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Active Support Core Extensions

Active Support is the Rails component responsible for providing Ruby language extensions, utilities, and other transversal stuff. It offers a richer bottom-line at the language level, targeted both at the development of Rails applications, and at the development of Rails itself.

By referring to this guide you will learn the extensions to the Ruby core classes and modules provided by Rails.


How to Load Core Extensions

Stand-Alone Active Support

In order to have a near zero default footprint, Active Support does not load anything by default. It is broken in small pieces so that you may load just what you need, and also has some convenience entry points to load related extensions in one shot, even everything.

Thus, after a simple require like:

require ‘active_support’

objects do not even respond to blank?, let’s see how to load its definition.

Cherry-picking a Definition

The most lightweight way to get blank? is to cherry-pick the file that defines it.

For every single method defined as a core extension this guide has a note that says where is such a method defined. In the case of blank? the note reads:

NOTE: Defined in active_support/core_ext/object/blank.rb.

That means that this single call is enough:

require ‘active_support/core_ext/object/blank’

Active Support has been carefully revised so that cherry-picking a file loads only strictly needed dependencies, if any.

Loading Grouped Core Extensions

The next level is to simply load all extensions to Object. As a rule of thumb, extensions to SomeClass are available in one shot by loading active_support/core_ext/some_class.

Thus, if that would do, to have blank? available we could just load all extensions to Object:

require ‘active_support/core_ext/object’

Loading All Core Extensions

You may prefer just to load all core extensions, there is a file for that:

require ‘active_support/core_ext’

Loading All Active Support

And finally, if you want to have all Active Support available just issue:

require ‘active_support/all’

That does not even put the entire Active Support in memory upfront indeed, some stuff is configured via autoload, so it is only loaded if used.

Active Support Within a Ruby on Rails Application

A Ruby on Rails application loads all Active Support unless config.active_support.bare is true. In that case, the application will only load what the framework itself cherry-picks for its own needs, and can still cherry-pick itself at any granularity level, as explained in the previous section.

Extensions to All Objects

blank? and present?

The following values are considered to be blank in a Rails application:

  • nil and false,
  • strings composed only of whitespace, i.e. matching /\A\s*\z/,
  • empty arrays and hashes, and
  • any other object that responds to empty? and it is empty.

WARNING: Note that numbers are not mentioned, in particular 0 and 0.0 are not blank.

For example, this method from ActionDispatch::Response uses blank? to easily be robust to nil and whitespace strings in one shot:

def charset
charset = String(headers[“Content-Type”] || headers[“type”]).split(“;”)1
charset.blank? ? nil : charset.strip.split(“=”)1

That’s a typical use case for blank?.

Here, the method Rails runs to instantiate observers upon initialization has nothing to do if there are none:

def instantiate_observers
return if @observers.blank?

  1. end

The method present? is equivalent to !blank?:

assert response.body.present? # same as !response.body.blank?

NOTE: Defined in active_support/core_ext/object/blank.rb.


The presence method returns its receiver if present?, and nil otherwise. It is useful for idioms like this:

host = config[:host].presence || ‘localhost’

NOTE: Defined in active_support/core_ext/object/blank.rb.


A few fundamental objects in Ruby are singletons. For example, in the whole live of a program the integer 1 refers always to the same instance:

1.object_id # => 3
Math.cos(0).to_i.object_id # => 3

Hence, there’s no way these objects can be duplicated through dup or clone:

true.dup # => TypeError: can’t dup TrueClass

Some numbers which are not singletons are not duplicable either:

0.0.clone # => allocator undefined for Float
(2**1024).clone # => allocator undefined for Bignum

Active Support provides duplicable? to programmatically query an object about this property:

"".duplicable? # => true
false.duplicable? # => false

By definition all objects are duplicable? except nil, false, true, symbols, numbers, and class objects.

WARNING. Using duplicable? is discouraged because it depends on a hard-coded list. Classes have means to disallow duplication like removing dup and clone or raising exceptions from them, only rescue can tell.

NOTE: Defined in active_support/core_ext/object/duplicable.rb.


The method returning yields its argument to a block and returns it. You typically use it with a mutable object that gets modified in the block:

def html_options_for_form(url_for_options, options, *parameters_for_url)
returning options.stringify_keys do |html_options|
html_options[“enctype”] = “multipart/form-data” if html_options.delete(“multipart”)
html_options[“action”] = url_for(url_for_options, *parameters_for_url)

NOTE: Defined in active_support/core_ext/object/returning.rb.


Sometimes you want to call a method provided the receiver object is not nil, which is something you usually check first.

For instance, note how this method of ActiveRecord::ConnectionAdapters::AbstractAdapter checks if there’s a @logger:

def log_info(sql, name, ms)
if @logger && @logger.debug?
name = ‘s (.1fms)’ % [name || ‘SQL’, ms]
@logger.debug(format_log_entry(name, sql.squeeze(’ ’)))

You can shorten that using Object#try. This method is a synonym for Object#send except that it returns nil if sent to nil. The previous example could then be rewritten as:

def log_info(sql, name, ms)
if @logger.try(:debug?)
name = ‘s (.1fms)’ % [name || ‘SQL’, ms]
@logger.debug(format_log_entry(name, sql.squeeze(’ ’)))

NOTE: Defined in active_support/core_ext/object/try.rb.


The method singleton_class returns the singleton class of the receiver:

String.singleton_class # => # # => #<Class:#>

WARNING: Fixnums and symbols have no singleton classes, singleton_class
raises TypeError on them. Moreover, the singleton classes of nil, true, and false, are NilClass, TrueClass, and FalseClass, respectively.

NOTE: Defined in active_support/core_ext/kernel/singleton_class.rb.

class_eval(*args, &block)

You can evaluate code in the context of any object’s singleton class using class_eval:

class Proc
def bind(object)
block, time = self,
object.class_eval do
method_name = “__bind_#{time.to_i}_#{time.usec}”
define_method(method_name, &block)
method = instance_method(method_name)

NOTE: Defined in active_support/core_ext/kernel/singleton_class.rb.


The method acts_like provides a way to check whether some class acts like some other class based on a simple convention: a class that provides the same interface as String defines

def acts_like_string?

which is only a marker, its body or return value are irrelevant. Then, client code can query for duck-type-safeness this way:


Rails has classes that act like Date or Time and follow this contract.

NOTE: Defined in active_support/core_ext/object/acts_like.rb.


All objects in Rails respond to the method to_param, which is meant to return something that represents them as values in a query string, or as a URL fragments.

By default to_param just calls to_s:

7.to_param # => “7”

The return value of to_param should not be escaped:

“Tom & Jerry”.to_param # => “Tom & Jerry”

Several classes in Rails overwrite this method.

For example nil, true, and false return themselves. Array#to_param calls to_param on the elements and joins the result with “/”:

[0, true, String].to_param # => “0/true/String”

Notably, the Rails routing system calls to_param on models to get a value for the :id placeholder. ActiveRecord::Base#to_param returns the id of a model, but you can redefine that method in your models. For example, given

class User
def to_param

we get:

user_path(@user) # => “/users/357-john-smith”

WARNING. Controllers need to be aware of any redefinition of to_param because when a request like that comes in “357-john-smith” is the value of params[:id].

NOTE: Defined in active_support/core_ext/object/to_param.rb.


Except for hashes, given an unescaped key this method constructs the part of a query string that would map such key to what to_param returns. For example, given

class User
def to_param

we get:

current_user.to_query(‘user’) # => user=357-john-smith

This method escapes whatever is needed, both for the key and the value:


  1. => “company%5Bname%5D=Johnson+%26+Johnson”

so its output is ready to be used in a query string.

Arrays return the result of applying to_query to each element with key[] as key, and join the result with “&”:

[3.4, -45.6].to_query(‘sample’)

  1. => “sample%5B%5D=3.4&sample%5B%5D=-45.6”

Hashes also respond to to_query but with a different signature. If no argument is passed a call generates a sorted series of key/value assignments calling to_query(key) on its values. Then it joins the result with “&”:

{:c => 3, :b => 2, :a => 1}.to_query # => “a=1&b=2&c=3”

The method Hash#to_query accepts an optional namespace for the keys:

{:id => 89, :name => “John Smith”}.to_query(‘user’)

  1. => “user%5Bid%5D=89&user%5Bname%5D=John+Smith”

NOTE: Defined in active_support/core_ext/object/to_query.rb.


The method with_options provides a way to factor out common options in a series of method calls.

Given a default options hash, with_options yields a proxy object to a block. Within the block, methods called on the proxy are forwarded to the receiver with their options merged. For example, you get rid of the duplication in:

class Account < ActiveRecord::Base
has_many :customers, :dependent => :destroy
has_many :products, :dependent => :destroy
has_many :invoices, :dependent => :destroy
has_many :expenses, :dependent => :destroy

this way:

class Account < ActiveRecord::Base
with_options :dependent => :destroy do |assoc|
assoc.has_many :customers
assoc.has_many :products
assoc.has_many :invoices
assoc.has_many :expenses

That idiom may convey grouping to the reader as well. For example, say you want to send a newsletter whose language depends on the user. Somewhere in the mailer you could group locale-dependent bits like this:

I18n.with_options :locale => user.locale, :scope => “newsletter” do |i18n|
subject i18n.t :subject
body i18n.t :body, :user_name =>

TIP: Since with_options forwards calls to its receiver they can be nested. Each nesting level will merge inherited defaults in addition to their own.

NOTE: Defined in active_support/core_ext/object/with_options.rb.


The method subclasses_of receives an arbitrary number of class objects and returns all their anonymous or reachable descendants as a single array:

class C; end
subclasses_of© # => []

subclasses_of(Integer) # => [Bignum, Fixnum]

module M
class A; end
class B1 < A; end
class B2 < A; end

module N
class C < M::B1; end

subclasses_of(M::A) # => [N::C, M::B2, M::B1]

The order in which these classes are returned is unspecified. The returned collection may have duplicates:

subclasses_of(Numeric, Integer)

  1. => [Bignum, Float, Fixnum, Integer, Date::Infinity, Rational, BigDecimal, Bignum, Fixnum]

See also Class#subclasses in Extensions to Class FIX THIS LINK.

NOTE: Defined in active_support/core_ext/object/extending.rb.

Instance Variables

Active Support provides several methods to ease access to instance variables.


Ruby 1.8 and 1.9 have a method called instance_variables that returns the names of the defined instance variables. But they behave differently, in 1.8 it returns strings whereas in 1.9 it returns symbols. Active Support defines instance_variable_names as a portable way to obtain them as strings:

class C
def initialize(x, y)
@x, @y = x, y
end, 1).instance_variable_names # => [“@y”, “@x”]

WARNING: The order in which the names are returned is unspecified, and it indeed depends on the version of the interpreter.

NOTE: Defined in active_support/core_ext/object/instance_variables.rb.


The method instance_values returns a hash that maps instance variable names without “@” to their
corresponding values. Keys are strings both in Ruby 1.8 and 1.9:

class C
def initialize(x, y)
@x, @y = x, y
end, 1).instance_values # => {"x" => 0, “y” => 1}

NOTE: Defined in active_support/core_ext/object/instance_variables.rb.

copy_instance_variables_from(object, exclude = [])

Copies the instance variables of object into self.

Instance variable names in the exclude array are ignored. If object
responds to protected_instance_variables the ones returned are
also ignored. For example, Rails controllers implement that method.

In both arrays strings and symbols are understood, and they have to include
the at sign.

class C
def initialize(x, y, z)
@x, @y, @z = x, y, z

def protected_instance_variables %w(@z) end


a =, 1, 2)
b =, 4, 5)

a.copy_instance_variables_from(b, [:@y])

  1. a is now: @x = 3, @y = 1, @z = 2

In the example object and self are of the same type, but they don’t need to.

NOTE: Defined in active_support/core_ext/object/instance_variables.rb.

Silencing Warnings, Streams, and Exceptions

The methods silence_warnings and enable_warnings change the value of $VERBOSE accordingly for the duration of their block, and reset it afterwards:

silence_warnings { Object.const_set “RAILS_DEFAULT_LOGGER”, logger }

You can silence any stream while a block runs with silence_stream:

silence_stream(STDOUT) do

  1. STDOUT is silent here

Silencing exceptions is also possible with suppress. This method receives an arbitrary number of exception classes. If an exception is raised during the execution of the block and is kind_of? any of the arguments, suppress captures it and returns silently. Otherwise the exception is reraised:

  1. If the user is locked the increment is lost, no big deal.
    suppress(ActiveRecord::StaleObjectError) do
    current_user.increment! :visits

NOTE: Defined in active_support/core_ext/kernel/reporting.rb.


The convenience method require_library_or_gem tries to load its argument with a regular require first. If it fails loads rubygems and tries again.

If the first attempt is a failure and rubygems can’t be loaded the method raises LoadError. On the other hand, if rubygems is available but the argument is not loadable as a gem, the method gives up and LoadError is also raised.

For example, that’s the way the MySQL adapter loads the MySQL library:


NOTE: Defined in active_support/core_ext/kernel/requires.rb.

Extensions to Module


Using plain Ruby you can wrap methods with other methods, that’s called alias chaining.

For example, let’s say you’d like params to be strings in functional tests, as they are in real requests, but still want the convenience of assigning integers and other kind of values. To accomplish that you could wrap ActionController::TestCase#process this way in test/test_helper.rb:

ActionController::TestCase.class_eval do

  1. save a reference to the original process method
    alias_method :original_process, :process
  1. now redefine process and delegate to original_process
    def process(action, params=nil, session=nil, flash=nil, http_method=‘GET’)
    params = Hash[* {|k, v| [k, v.to_s]}.flatten]
    original_process(action, params, session, flash, http_method)

That’s the method get, post, etc., delegate the work to.

That technique has a risk, it could be the case that :original_process was taken. To try to avoid collisions people choose some label that characterizes what the chaining is about:

ActionController::TestCase.class_eval do
def process_with_stringified_params(…)
params = Hash[* {|k, v| [k, v.to_s]}.flatten]
process_without_stringified_params(action, params, session, flash, http_method)
alias_method :process_without_stringified_params, :process
alias_method :process, :process_with_stringified_params

The method alias_method_chain provides a shortcut for that pattern:

ActionController::TestCase.class_eval do
def process_with_stringified_params(…)
params = Hash[* {|k, v| [k, v.to_s]}.flatten]
process_without_stringified_params(action, params, session, flash, http_method)
alias_method_chain :process, :stringified_params

Rails uses alias_method_chain all over the code base. For example validations are added to ActiveRecord::Base#save by wrapping the method that way in a separate module specialised in validations.

NOTE: Defined in active_support/core_ext/module/aliasing.rb.



Model attributes have a reader, a writer, and a predicate. You can aliase a model attribute having the corresponding three methods defined for you in one shot. As in other aliasing methods, the new name is the first argument, and the old name is the second (my mnemonic is they go in the same order as if you did an assignment):

class User < ActiveRecord::Base

  1. let me refer to the email column as “login”,
  2. much meaningful for authentication code
    alias_attribute :login, :email

NOTE: Defined in active_support/core_ext/module/aliasing.rb.


The method attr_accessor_with_default serves the same purpose as the Ruby macro attr_accessor but allows you to set a default value for the attribute:

class Url
attr_accessor_with_default :port, 80
end # => 80

The default value can be also specified with a block, which is called in the context of the corresponding object:

class User
attr_accessor :name, :surname
attr_accessor_with_default(:full_name) {
[name, surname].compact.join(" ")

u = = ‘Xavier’
u.surname = ‘Noria’
u.full_name # => “Xavier Noria”

The result is not cached, the block is invoked in each call to the reader.

You can overwrite the default with the writer:

url = # => 80 = 8080 # => 8080

The default value is returned as long as the attribute is unset. The reader does not rely on the value of the attribute to know whether it has to return the default. It rather monitors the writer: if there’s any assignment the value is no longer considered to be unset.

Active Resource uses this macro to set a default value for the :primary_key attribute:

attr_accessor_with_default :primary_key, ‘id’

NOTE: Defined in active_support/core_ext/module/attr_accessor_with_default.rb.

Internal Attributes

When you are defining an attribute in a class that is meant to be subclassed name collisions are a risk. That’s remarkably important for libraries.

Active Support defines the macros attr_internal_reader, attr_internal_writer, and attr_internal_accessor. They behave like their Ruby builtin attr_* counterparts, except they name the underlying instance variable in a way that makes collisions less likely.

The macro attr_internal is a synonym for attr_internal_accessor:

  1. library
    class ThirdPartyLibrary::Crawler
    attr_internal :log_level
  1. client code
    class MyCrawler < ThirdPartyLibrary::Crawler
    attr_accessor :log_level

In the previous example it could be the case that :log_level does not belong to the public interface of the library and it is only used for development. The client code, unaware of the potential conflict, subclasses and defines its own :log_level. Thanks to attr_internal there’s no collision.

By default the internal instance variable is named with a leading underscore, @_log_level in the example above. That’s configurable via Module.attr_internal_naming_format though, you can pass any sprintf-like format string with a leading @ and a %s somewhere, which is where the name will be placed. The default is “@_%s”.

Rails uses internal attributes in a few spots, for examples for views:

module ActionView
class Base
attr_internal :captures
attr_internal :request, :layout
attr_internal :controller, :template

NOTE: Defined in active_support/core_ext/module/attr_internal.rb.

Module Attributes

The macros mattr_reader, mattr_writer, and mattr_accessor are analogous to the cattr_* macros defined for class. Check Class Attributes.

For example, the dependencies mechanism uses them:

module ActiveSupport
module Dependencies
mattr_accessor :warnings_on_first_load
mattr_accessor :history
mattr_accessor :loaded
mattr_accessor :mechanism
mattr_accessor :load_paths
mattr_accessor :load_once_paths
mattr_accessor :autoloaded_constants
mattr_accessor :explicitly_unloadable_constants
mattr_accessor :logger
mattr_accessor :log_activity
mattr_accessor :constant_watch_stack
mattr_accessor :constant_watch_stack_mutex

NOTE: Defined in active_support/core_ext/module/attribute_accessors.rb.

Method Delegation

The class method delegate offers an easy way to forward methods.

For example, if User has some details like the age factored out to Profile, it could be handy to still be able to access such attributes directly, user.age, instead of having to explicit the chain user.profile.age.

That can be accomplished by hand:

class User
has_one :profile

def age profile.age end


But with delegate you can make that shorter and the intention even more obvious:

class User
has_one :profile

delegate :age, to => :profile


The macro accepts more than one method:

class User
has_one :profile

delegate :age, :avatar, :twitter_username, to => :profile


Methods can be delegated to objects returned by methods, as in the examples above, but also to instance variables, class variables, and constants. Just pass their names as symbols or strings, including the at signs in the last cases.

For example, ActionView::Base delegates erb_trim_mode=:

module ActionView
class Base
delegate :erb_trim_mode=, :to => ‘ActionView::Template::Handlers::ERB’

In fact, you can delegate to any expression passed as a string. It will be evaluated in the context of the receiver. Controllers for example delegate alerts and notices to the current flash:

delegate :alert, :notice, :to => “request.flash”

If the target is nil calling any delegated method will raise an exception even if nil responds to such method. You can override this behaviour setting the option :allow_nil to true, in which case the forwarded call will simply return nil.

If the target is a method, the name of delegated methods can also be prefixed. If the :prefix option is set to (exactly) the true object, the value of the :to option is prefixed:

class Invoice
belongs_to :customer

  1. defines a method called customer_name
    delegate :name, :to => :customer, :prefix => true

And a custom prefix can be set as well, in that case it does not matter wheter the target is a method or not:

class Account
belongs_to :user

  1. defines a method called admin_email
    delegate :email, :to => :user, :prefix => ‘admin’

NOTE: Defined in active_support/core_ext/module/delegation.rb.

Method Removal


The method remove_possible_method is like the standard remove_method, except it silently returns on failure:

class A; end

A.class_eval do
remove_method(:nonexistent) # raises NameError
remove_possible_method(:nonexistent) # no problem, continue

This may come in handy if you need to define a method that may already exist, since redefining a method issues a warning “method redefined; discarding old redefined_method_name”.

NOTE: Defined in active_support/core_ext/module/remove_method.rb.



The parent method on a nested named module returns the module that contains its corresponding constant:

module X
module Y
module Z
M = X::Y::Z

X::Y::Z.parent # => X::Y
M.parent # => X::Y

If the module is anonymous or belongs to the top-level, parent returns Object.

WARNING: Note that in that case parent_name returns nil.

NOTE: Defined in active_support/core_ext/module/introspection.rb.


The parent_name method on a nested named module returns the fully-qualified name of the module that contains its corresponding constant:

module X
module Y
module Z
M = X::Y::Z

X::Y::Z.parent_name # => “X::Y”
M.parent_name # => “X::Y”

For top-level or anonymous modules parent_name returns nil.

WARNING: Note that in that case parent returns Object.

NOTE: Defined in active_support/core_ext/module/introspection.rb.


The method parents calls parent on the receiver and upwards until Object is reached. The chain is returned in an array, from bottom to top:

module X
module Y
module Z
M = X::Y::Z

X::Y::Z.parents # => [X::Y, X, Object]
M.parents # => [X::Y, X, Object]

NOTE: Defined in active_support/core_ext/module/introspection.rb.


The method local_constants returns the names of the constants that have been defined in the receiver module:

module X
X1 = 1
X2 = 2
module Y
Y1 = :y1
X1 = :overrides_X1_above

X.local_constants # => [“X2”, “X1”, “Y”], assumes Ruby 1.8
X::Y.local_constants # => [“X1”, “Y1”], assumes Ruby 1.8

The names are returned as strings in Ruby 1.8, and as symbols in Ruby 1.9. The method local_constant_names returns always strings.

WARNING: This method is exact if running under Ruby 1.9. In previous versions it may miss some constants if their value in some ancestor stores the exact same object than in the receiver.

NOTE: Defined in active_support/core_ext/module/introspection.rb.


The synchronize macro declares a method to be synchronized:

class Counter
@@mutex =
attr_reader :value

def initialize @value = 0 end def incr @value += 1 # non-atomic end synchronize :incr, :with => ‘@@mutex’


The method receives the name of an action, and a :with option with code. The code is evaluated in the context of the receiver each time the method is invoked, and it should evaluate to a Mutex instance or any other object that responds to synchronize and accepts a block.

NOTE: Defined in active_support/core_ext/module/synchronization.rb.


A named module is reachable if it is stored in its corresponding constant. It means you can reach the module object via the constant.

That is what ordinarily happens, if a module is called “M”, the M constant exists and holds it:

module M

M.reachable? # => true

But since constants and modules are indeed kind of decoupled, module objects can become unreachable:

module M

orphan = Object.send(:remove_const, :M)

  1. The module object is orphan now but it still has a name. # => “M”
  1. You cannot reach it via the constant M because it does not even exist.
    orphan.reachable? # => false
  1. Let’s define a module called “M” again.
    module M
  1. The constant M exists now again, and it stores a module
  2. object called “M”, but it is a new instance.
    orphan.reachable? # => false

NOTE: Defined in active_support/core_ext/module/reachable.rb.


A module may or may not have a name:

module M
end # => “M”

N = # => “N” # => "" in 1.8, nil in 1.9

You can check whether a module has a name with the predicate anonymous?:

module M
M.anonymous? # => false # => true

Note that being unreachable does not imply being anonymous:

module M

m = Object.send(:remove_const, :M)

m.reachable? # => false
m.anonymous? # => false

though an anonymous module is unreachable by definition.

NOTE: Defined in active_support/core_ext/module/anonymous.rb.

Extensions to Class

Class Attributes

The method Class#class_attribute declares one or more inheritable class attributes that can be overridden at any level down the hierarchy:

class A
class_attribute :x

class B < A; end

class C < B; end

A.x = :a
B.x # => :a
C.x # => :a

B.x = :b
A.x # => :a
C.x # => :b

C.x = :c
A.x # => :a
B.x # => :b

For example that’s the way the allow_forgery_protection flag is implemented for controllers:

class_attribute :allow_forgery_protection
self.allow_forgery_protection = true

For convenience class_attribute defines also a predicate, so that declaration also generates allow_forgery_protection?. Such predicate returns the double boolean negation of the value.

NOTE: Defined in active_support/core_ext/class/attribute.rb

The macros cattr_reader, cattr_writer, and cattr_accessor are analogous to their attr_* counterparts but for classes. They initialize a class variable to nil unless it already exists, and generate the corresponding class methods to access it:

class MysqlAdapter < AbstractAdapter

  1. Generates class methods to access @@emulate_booleans.
    cattr_accessor :emulate_booleans
    self.emulate_booleans = true

Instance methods are created as well for convenience. For example given

module ActionController
class Base
cattr_accessor :logger

we can access logger in actions. The generation of the writer instance method can be prevented setting :instance_writer to false (not any false value, but exactly false):

module ActiveRecord
class Base

  1. No pluralize_table_names= instance writer is generated.
    cattr_accessor :pluralize_table_names, :instance_writer => false

A model may find that option useful as a way to prevent mass-assignment from setting the attribute.

NOTE: Defined in active_support/core_ext/class/attribute_accessors.rb.

Class Inheritable Attributes

Class variables are shared down the inheritance tree. Class instance variables are not shared, but they are not inherited either. The macros class_inheritable_reader, class_inheritable_writer, and class_inheritable_accessor provide accessors for class-level data which is inherited but not shared with children:

module ActionController
class Base

  2. The value of allow_forgery_protection is inherited,
  3. but its value in a particular class does not affect
  4. the value in the rest of the controllers hierarchy.
    class_inheritable_accessor :allow_forgery_protection

They accomplish this with class instance variables and cloning on subclassing, there are no class variables involved. Cloning is performed with dup as long as the value is duplicable.

There are some variants specialised in arrays and hashes:


Those writers take any inherited array or hash into account and extend them rather than overwrite them.

As with vanilla class attribute accessors these macros create convenience instance methods for reading and writing. The generation of the writer instance method can be prevented setting :instance_writer to false (not any false value, but exactly false):

module ActiveRecord
class Base
class_inheritable_accessor :default_scoping, :instance_writer => false

Since values are copied when a subclass is defined, if the base class changes the attribute after that, the subclass does not see the new value. That’s the point.

NOTE: Defined in active_support/core_ext/class/inheritable_attributes.rb.

There’s a related macro called superclass_delegating_accessor, however, that does not copy the value when the base class is subclassed. Instead, it delegates reading to the superclass as long as the attribute is not set via its own writer. For example, ActionMailer::Base defines delivery_method this way:

module ActionMailer
class Base
superclass_delegating_accessor :delivery_method
self.delivery_method = :smtp

If for whatever reason an application loads the definition of a mailer class and after that sets ActionMailer::Base.delivery_method, the mailer class will still see the new value. In addition, the mailer class is able to change the delivery_method without affecting the value in the parent using its own inherited class attribute writer.

NOTE: Defined in active_support/core_ext/class/delegating_attributes.rb.



The subclasses method returns the names of all the anonymous or reachable descendants of its receiver as an array of strings:

class C; end
C.subclasses # => []

Integer.subclasses # => [“Bignum”, “Fixnum”]

module M
class A; end
class B1 < A; end
class B2 < A; end

module N
class C < M::B1; end

M::A.subclasses # => [“N::C”, “M::B2”, “M::B1”]

The order in which these class names are returned is unspecified.

See also Object#subclasses_of in Extensions to All Objects FIX THIS LINK.

WARNING: This method is redefined in some Rails core classes.

NOTE: Defined in active_support/core_ext/class/subclasses.rb.

Extensions to String

Output Safety

Inserting data into HTML templates needs extra care. For example you can’t just interpolate @review.title verbatim into an HTML page. On one hand if the review title is “Flanagan & Matz rules!” the output won’t be well-formed because an ampersand has to be escaped as “&amp;”. On the other hand, depending on the application that may be a big security hole because users can inject malicious HTML setting a hand-crafted review title. Check out the section about cross-site scripting in the Security guide for further information about the risks.

Active Support has the concept of (html) safe strings since Rails 3. A safe string is one that is marked as being insertable into HTML as is. It is trusted, no matter whether it has been escaped or not.

Strings are considered to be unsafe by default:

"".html_safe? # => false

You can obtain a safe string from a given one with the html_safe method:

s = "".html_safe
s.html_safe? # => true

It is important to understand that html_safe performs no escaping whatsoever, it is just an assertion:

s = “”.html_safe
s.html_safe? # => true
s # => “”

It is your responsibility to ensure calling html_safe on a particular string is fine.

NOTE: For performance reasons safe strings are implemented in a way that cannot offer an in-place html_safe! variant.

If you append onto a safe string, either in-place with concat/<<, or with +, the result is a safe string. Unsafe arguments are escaped:

"".html_safe + “<” # => “<”

Safe arguments are directly appended:

"".html_safe + “<”.html_safe # => “<”

These methods should not be used in ordinary views. In Rails 3 unsafe values are automatically escaped:

<%= @review.title > <# fine in Rails 3, escaped if needed %>

To insert something verbatim use the raw helper rather than calling html_safe:

<%= raw @cms.current_template > <# inserts @cms.current_template as is %>

The raw helper calls html_safe for you:

def raw(stringish)

NOTE: Defined in active_support/core_ext/string/output_safety.rb.


The method String#squish strips leading and trailing whitespace, and substitutes runs of whitespace with a single space each:

" \n foo\n\r \t bar \n".squish # => “foo bar”

There’s also the destructive version String#squish!.

NOTE: Defined in active_support/core_ext/string/filters.rb.


The method truncate returns a copy of its receiver truncated after a given length:

“Oh dear! Oh dear! I shall be late!”.truncate(20)

  1. => “Oh dear! Oh dear!…”

Ellipsis can be customized with the :omission option:

“Oh dear! Oh dear! I shall be late!”.truncate(20, :omission => ‘…’)

  1. => “Oh dear! Oh …”

Note in particular that truncation takes into account the length of the omission string.

Pass a :separator to truncate the string at a natural break:

“Oh dear! Oh dear! I shall be late!”.truncate(18)

  1. => “Oh dear! Oh dea…”
    “Oh dear! Oh dear! I shall be late!”.truncate(18, :separator => ’ ’)
  2. => “Oh dear! Oh…”

In the above example “dear” gets cut first, but then :separator prevents it.

WARNING: The option :separator can’t be a regexp.

NOTE: Defined in active_support/core_ext/string/filters.rb.

Key-based Interpolation

In Ruby 1.9 the % string operator supports key-based interpolation, both formatted and unformatted:

“Total is %.02f” % {:total => 43.1} # => Total is 43.10
“I say %{foo}” % {:foo => "wadus"} # => “I say wadus”
“I say %{woo}” % {:foo => "wadus"} # => KeyError

Active Support adds that functionality to % in previous versions of Ruby.

NOTE: Defined in active_support/core_ext/string/interpolation.rb.

starts_with? and ends_width?

Active Support defines 3rd person aliases of String#start_with? and String#end_with?:

“foo”.starts_with?(“f”) # => true
“foo”.ends_with?(“o”) # => true

NOTE: Defined in active_support/core_ext/string/starts_ends_with.rb.



Returns the character of the string at position position:

“hello”.at(0) # => “h”
“hello”.at(4) # => “o”
“hello”.at(-1) # => “o”
“hello”.at(10) # => ERROR if < 1.9, nil in 1.9

NOTE: Defined in active_support/core_ext/string/access.rb.


Returns the substring of the string starting at position position:

“hello”.from(0) # => “hello”
“hello”.from(2) # => “llo”
“hello”.from(-2) # => “lo”
“hello”.from(10) # => "" if < 1.9, nil in 1.9

NOTE: Defined in active_support/core_ext/string/access.rb.


Returns the substring of the string up to position position:

“hello”.to(0) # => “h”
“hello”.to(2) # => “hel”
“hello”.to(-2) # => “hell”
“hello”.to(10) # => “hello”

NOTE: Defined in active_support/core_ext/string/access.rb.

first(limit = 1)

The call str.first(n) is equivalent to if n > 0, and returns an empty string for n == 0.

NOTE: Defined in active_support/core_ext/string/access.rb.

last(limit = 1)

The call str.last(n) is equivalent to str.from(-n) if n > 0, and returns an empty string for n == 0.

NOTE: Defined in active_support/core_ext/string/access.rb.



The method pluralize returns the plural of its receiver:

“table”.pluralize # => “tables”
“ruby”.pluralize # => “rubies”
“equipment”.pluralize # => “equipment”

As the previous example shows, Active Support knows some irregular plurals and uncountable nouns. Built-in rules can be extended in config/initializers/inflections.rb. That file is generated by the rails command and has instructions in comments.

Active Record uses this method to compute the default table name that corresponds to a model:

  1. active_record/base.rb
    def undecorated_table_name(class_name =
    table_name = class_name.to_s.demodulize.underscore
    table_name = table_name.pluralize if pluralize_table_names

NOTE: Defined in active_support/core_ext/string/inflections.rb.


The inverse of pluralize:

“tables”.singularize # => “table”
“rubies”.singularize # => “ruby”
“equipment”.singularize # => “equipment”

Associations compute the name of the corresponding default associated class using this method:

  1. active_record/reflection.rb
    def derive_class_name
    class_name = name.to_s.camelize
    class_name = class_name.singularize if collection?

NOTE: Defined in active_support/core_ext/string/inflections.rb.


The method camelize returns its receiver in camel case:

“product”.camelize # => “Product”
“admin_user”.camelize # => “AdminUser”

As a rule of thumb you can think of this method as the one that transforms paths into Ruby class or module names, where slashes separate namespaces:

“backoffice/session”.camelize # => “Backoffice::Session”

For example, Action Pack uses this method to load the class that provides a certain session store:

  1. action_controller/metal/session_management.rb
    def session_store=(store)
    if store == :active_record_store
    self.session_store = ActiveRecord::SessionStore
    @@session_store = store.is_a?(Symbol) ?
    ActionDispatch::Session.const_get(store.to_s.camelize) :

camelize accepts an optional argument, it can be :upper (default), or :lower. With the latter the first letter becomes lowercase:

“visual_effect”.camelize(:lower) # => “visualEffect”

That may be handy to compute method names in a language that follows that convention, for example JavaScript.

camelize is aliased to camelcase.

NOTE: Defined in active_support/core_ext/string/inflections.rb.


The method underscore is the inverse of camelize, explained above:

“Product”.underscore # => “product”
“AdminUser”.underscore # => “admin_user”

Also converts “::” back to “/”:

“Backoffice::Session”.underscore # => “backoffice/session”

and understands strings that start with lowercase:

“visualEffect”.underscore # => “visual_effect”

underscore accepts no argument though.

Rails class and module autoloading uses underscore to infer the relative path without extension of a file that would define a given missing constant:

  1. active_support/dependencies.rb
    def load_missing_constant(from_mod, const_name)

    qualified_name = qualified_name_for from_mod, const_name
    path_suffix = qualified_name.underscore


NOTE: Defined in active_support/core_ext/string/inflections.rb.


The method titleize capitalizes the words in the receiver:

“alice in wonderland”.titleize # => “Alice In Wonderland”
“fermat’s enigma”.titleize # => “Fermat’s Enigma”

titleize is aliased to titlecase.

NOTE: Defined in active_support/core_ext/string/inflections.rb.


The method dasherize replaces the underscores in the receiver with dashes:

“name”.dasherize # => “name”
“contact_data”.dasherize # => “contact-data”

The XML serializer of models uses this method to dasherize node names:

  1. active_model/serializers/xml.rb
    def reformat_name(name)
    name = name.camelize if camelize?
    dasherize? ? name.dasherize : name

NOTE: Defined in active_support/core_ext/string/inflections.rb.


Given a string with a qualified constant reference expression, demodulize returns the very constant name, that is, the rightmost part of it:

“Product”.demodulize # => “Product”
“Backoffice::UsersController”.demodulize # => “UsersController”
“Admin::Hotel::ReservationUtils”.demodulize # => “ReservationUtils”

Active Record for example uses this method to compute the name of a counter cache column:

  1. active_record/reflection.rb
    def counter_cache_column
    if options[:counter_cache] == true
    elsif options[:counter_cache]

NOTE: Defined in active_support/core_ext/string/inflections.rb.


The method parameterize normalizes its receiver in a way that can be used in pretty URLs.

“John Smith”.parameterize # => “john-smith”
“Kurt Gödel”.parameterize # => “kurt-godel”

In fact, the result string is wrapped in an instance of ActiveSupport::Multibyte::Chars.

NOTE: Defined in active_support/core_ext/string/inflections.rb.


The method tableize is underscore followed by pluralize.

“Person”.tableize # => “people”
“Invoice”.tableize # => “invoices”
“InvoiceLine”.tableize # => “invoice_lines”

As a rule of thumb, tableize returns the table name that corresponds to a given model for simple cases. The actual implementation in Active Record is not straight tableize indeed, because it also demodulizes de class name and checks a few options that may affect the returned string.

NOTE: Defined in active_support/core_ext/string/inflections.rb.


The method classify is the inverse of tableize. It gives you the class name corresponding to a table name:

“people”.classify # => “Person”
“invoices”.classify # => “Invoice”
“invoice_lines”.classify # => “InvoiceLine”

The method understands qualified table names:

“highrise_production.companies”.classify # => “Company”

Note that classify returns a class name as a string. You can get the actual class object invoking constantize on it, explained next.

NOTE: Defined in active_support/core_ext/string/inflections.rb.


The method constantize resolves the constant reference expression in its receiver:

“Fixnum”.constantize # => Fixnum

module M
X = 1
“M::X”.constantize # => 1

If the string evaluates to no known constant, or its content is not even a valid constant name, constantize raises NameError.

Constant name resolution by constantize starts always at the top-level Object even if there is no leading “::”.

X = :in_Object
module M
X = :in_M

X # => :in_M “::X”.constantize # => :in_Object “X”.constantize # => :in_Object (!)


So, it is in general not equivalent to what Ruby would do in the same spot, had a real constant be evaluated.

Mailer test cases obtain the mailer being tested from the name of the test class using constantize:

  1. action_mailer/test_case.rb
    def determine_default_mailer(name)
    name.sub(/Test$/, ’’).constantize
    rescue NameError => e

NOTE: Defined in active_support/core_ext/string/inflections.rb.


The method humanize gives you a sensible name for display out of an attribute name. To do so it replaces underscores with spaces, removes any “_id” suffix, and capitalizes the first word:

“name”.humanize # => “Name”
“author_id”.humanize # => “Author”
“comments_count”.humanize # => “Comments count”

The helper method full_messages uses humanize as a fallback to include attribute names:

def full_messages
full_messages = []

each do |attribute, messages| … attr_name = attribute.to_s.gsub(‘.’, ‘_’).humanize attr_name = @base.class.human_attribute_name(attribute, :default => attr_name) … end full_messages


NOTE: Defined in active_support/core_ext/string/inflections.rb.


The method foreign_key gives a foreign key column name from a class name. To do so it demodulizes, underscores, and adds “_id”:

“User”.foreign_key # => “user_id”
“InvoiceLine”.foreign_key # => “invoice_line_id”
“Admin::Session”.foreign_key # => “session_id”

Pass a false argument if you do not want the underscore in “_id”:

“User”.foreign_key(false) # => “userid”

Associations use this method to infer foreign keys, for example has_one and has_many do this:

  1. active_record/associations.rb
    foreign_key = options[:foreign_key] ||

NOTE: Defined in active_support/core_ext/string/inflections.rb.

Extensions to Numeric


All numbers respond to these methods:


They return the corresponding amount of bytes, using a conversion factor of 1024:

2.kilobytes # => 2048
3.megabytes # => 3145728
3.5.gigabytes # => 3758096384
-4.exabytes # => -4611686018427387904

Singular forms are aliased so you are able to say:

1.megabyte # => 1048576

NOTE: Defined in active_support/core_ext/numeric/bytes.rb.

Extensions to Integer


The method multiple_of? tests whether an integer is multiple of the argument:

2.multiple_of?(1) # => true
1.multiple_of?(2) # => false

NOTE: Defined in active_support/core_ext/integer/multiple.rb.


The method ordinalize returns the ordinal string corresponding to the receiver integer:

1.ordinalize # => “1st”
2.ordinalize # => “2nd”
53.ordinalize # => “53rd”
2009.ordinalize # => “2009th”

NOTE: Defined in active_support/core_ext/integer/inflections.rb.

Extensions to Float


The built-in method Float#round rounds a float to the nearest integer. Active Support adds an optional parameter to let you specify a precision:

Math::E.round(4) # => 2.7183

NOTE: Defined in active_support/core_ext/float/rounding.rb.

Extensions to BigDecimal

Extensions to Enumerable


Ruby 1.8.7 and up define group_by, and Active Support does it for previous versions.

This iterator takes a block and builds an ordered hash with its return values as keys. Each key is mapped to the array of elements for which the block returned that value:

entries_by_surname_initial = address_book.group_by do |entry|

WARNING. Active Support redefines group_by in Ruby 1.8.7 so that it still returns an ordered hash.

NOTE: Defined in active_support/core_ext/enumerable.rb.


The method sum adds the elements of an enumerable:

[1, 2, 3].sum # => 6
(1..100).sum # => 5050

Addition only assumes the elements respond to +:

[[1, 2], [2, 3], [3, 4]].sum # => [1, 2, 2, 3, 3, 4]
%w(foo bar baz).sum # => “foobarbaz”
{:a => 1, :b => 2, :c => 3}.sum # => [:b, 2, :c, 3, :a, 1]

The sum of an empty collection is zero by default, but this is customizable:

[].sum # => 0
[].sum(1) # => 1

If a block is given sum becomes an iterator that yields the elements of the collection and sums the returned values:

(1..5).sum {|n| n * 2 } # => 30
[2, 4, 6, 8, 10].sum # => 30

The sum of an empty receiver can be customized in this form as well:

[].sum(1) {|n| n**3} # => 1

The method ActiveRecord::Observer#observed_subclasses for example is implemented this way:

def observed_subclasses
observed_classes.sum([]) { |klass| klass.send(:subclasses) }

NOTE: Defined in active_support/core_ext/enumerable.rb.


The inject method offers iteration with an accumulator:

[2, 3, 4].inject(1) {|acc, i| product*i } # => 24

The block is expected to return the value for the accumulator in the next iteration, and this makes building mutable objects a bit cumbersome:

[1, 2].inject({}) {|h, i| h[i] = i**2; h} # => {1 => 1, 2 => 4}

See that spurious “; h”?

Active Support backports each_with_object from Ruby 1.9, which addresses that use case. It iterates over the collection, passes the accumulator, and returns the accumulator when done. You normally modify the accumulator in place. The example above would be written this way:

[1, 2].each_with_object({}) {|i, h| h[i] = i**2} # => {1 => 1, 2 => 4}

WARNING. Note that the item of the collection and the accumulator come in different order in inject and each_with_object.

NOTE: Defined in active_support/core_ext/enumerable.rb.


The method index_by generates a hash with the elements of an enumerable indexed by some key.

It iterates through the collection and passes each element to a block. The element will be keyed by the value returned by the block:


  1. => {’2009-032’ => , ‘2009-008’ => , …}

WARNING. Keys should normally be unique. If the block returns the same value for different elements no collection is built for that key. The last item will win.

NOTE: Defined in active_support/core_ext/enumerable.rb.


The method many? is shorthand for collection.size > 1:

<% if pages.many? >
<= pagination_links >
< end %>

If an optional block is given many? only takes into account those elements that return true:

@see_more = videos.many? {|video| video.category == params[:category]}

NOTE: Defined in active_support/core_ext/enumerable.rb.


The predicate exclude? tests whether a given object does not belong to the collection. It is the negation of the builtin include?:

to_visit << node if visited.exclude?(node)

NOTE: Defined in active_support/core_ext/enumerable.rb.

Extensions to Array


Active Support augments the API of arrays to ease certain ways of accessing them. For example, to returns the subarray of elements up to the one at the passed index:

%w(a b c d).to(2) # => %w(a b c)
[].to(7) # => []

Similarly, from returns the tail from the element at the passed index on:

%w(a b c d).from(2) # => %w(c d)
%w(a b c d).from(10) # => nil
[].from(0) # => []

The methods second, third, fourth, and fifth return the corresponding element (first is built-in). Thanks to social wisdom and positive constructiveness all around, forty_two is also available.

NOTE: Defined in active_support/core_ext/array/access.rb.

Random Access

Active Support backports sample from Ruby 1.9:

shape_type = [Circle, Square, Triangle].sample

  1. => Square, for example

shape_types = [Circle, Square, Triangle].sample(2)

  1. => [Triangle, Circle], for example

NOTE: Defined in active_support/core_ext/array/random_access.rb.

Options Extraction

When the last argument in a method call is a hash, except perhaps for a &block argument, Ruby allows you to omit the brackets:

User.exists?(:email => params[:email])

That syntactic sugar is used a lot in Rails to avoid positional arguments where there would be too many, offering instead interfaces that emulate named parameters. In particular it is very idiomatic to use a trailing hash for options.

If a method expects a variable number of arguments and uses * in its declaration, however, such an options hash ends up being an item of the array of arguments, where kind of loses its role.

In those cases, you may give an options hash a distinguished treatment with extract_options!. That method checks the type of the last item of an array. If it is a hash it pops it and returns it, otherwise returns an empty hash.

Let’s see for example the definition of the caches_action controller macro:

def caches_action(*actions)
return unless cache_configured?
options = actions.extract_options!


This method receives an arbitrary number of action names, and an optional hash of options as last argument. With the call to extract_options! you obtain the options hash and remove it from actions in a simple and explicit way.

NOTE: Defined in active_support/core_ext/array/extract_options.rb.



The method to_sentence turns an array into a string containing a sentence that enumerates its items:

%w().to_sentence # => ""
%w(Earth).to_sentence # => “Earth”
%w(Earth Wind).to_sentence # => “Earth and Wind”
%w(Earth Wind Fire).to_sentence # => “Earth, Wind, and Fire”

This method accepts three options:

  • :two_words_connector: What is used for arrays of length 2. Default is " and ".
  • :words_connector: What is used to join the elements of arrays with 3 or more elements, except for the last two. Default is ", ".
  • :last_word_connector: What is used to join the last items of an array with 3 or more elements. Default is ", and ".

The defaults for these options can be localised, their keys are:

Option I18n key
:two_words_connector support.array.two_words_connector
:words_connector support.array.words_connector
:last_word_connector support.array.last_word_connector

Options :connector and :skip_last_comma are deprecated.

NOTE: Defined in active_support/core_ext/array/conversions.rb.


The method to_formatted_s acts like to_s by default.

If the array contains items that respond to id, however, it may be passed the symbol :db as argument. That’s typically used with collections of ARs, though technically any object in Ruby 1.8 responds to id indeed. Returned strings are:

[].to_formatted_s(:db) # => “null”
[user].to_formatted_s(:db) # => “8456”
invoice.lines.to_formatted_s(:db) # => “23,567,556,12”

Integers in the example above are supposed to come from the respective calls to id.

NOTE: Defined in active_support/core_ext/array/conversions.rb.


The method to_xml returns a string containing an XML representation of its receiver:

Contributor.all(:limit => 2, :order => ‘rank ASC’).to_xml

  1. =>
  2. <?xml version=“1.0” encoding=“UTF-8”?>
  3. 4356
  4. Jeremy Kemper
  5. 1
  6. jeremy-kemper
  7. 4404
  8. David Heinemeier Hansson
  9. 2
  10. david-heinemeier-hansson

To do so it sends to_xml to every item in turn, and collects the results under a root node. All items must respond to to_xml, an exception is raised otherwise.

By default, the name of the root element is the underscorized and dasherized plural of the name of the class of the first item, provided the rest of elements belong to that type (checked with is_a?) and they are not hashes. In the example above that’s “contributors”.

If there’s any element that does not belong to the type of the first one the root node becomes “records”:

[Contributor.first, Commit.first].to_xml

  1. =>
  2. <?xml version=“1.0” encoding=“UTF-8”?>
  3. 4583
  4. Aaron Batalion
  5. 53
  6. aaron-batalion
  7. Joshua Peek
  8. 2009-09-02T16:44:36Z
  9. origin/master
  10. 2009-09-02T16:44:36Z
  11. Joshua Peek
  12. 190316
  13. false
  14. Kill AMo observing wrap_with_notifications since ARes was only using it
  15. 723a47bfb3708f968821bc969a9a3fc873a3ed58

If the receiver is an array of hashes the root element is by default also “records”:

[{:a => 1, :b => 2}, {:c => 3}].to_xml

  1. =>
  2. <?xml version=“1.0” encoding=“UTF-8”?>
  3. 2
  4. 1
  5. 3

WARNING. If the collection is empty the root element is by default “nil-classes”. That’s a gotcha, for example the root element of the list of contributors above would not be “contributors” if the collection was empty, but “nil-classes”. You may use the :root option to ensure a consistent root element.

The name of children nodes is by default the name of the root node singularized. In the examples above we’ve seen “contributor” and “record”. The option :children allows you to set these node names.

The default XML builder is a fresh instance of Builder::XmlMarkup. You can configure your own builder via the :builder option. The method also accepts options like :dasherize and friends, they are forwarded to the builder:

Contributor.all(:limit => 2, :order => ‘rank ASC’).to_xml(:skip_types => true)

  1. =>
  2. <?xml version=“1.0” encoding=“UTF-8”?>
  3. 4356
  4. Jeremy Kemper
  5. 1
  6. jeremy-kemper
  7. 4404
  8. David Heinemeier Hansson
  9. 2
  10. david-heinemeier-hansson

NOTE: Defined in active_support/core_ext/array/conversions.rb.


The class method Array.wrap behaves like the function Array() except that it does not try to call to_a on its argument. That changes the behaviour for enumerables:

Array.wrap(:foo => :bar) # => [{:foo => :bar}]
Array(:foo => :bar) # => [[:foo, :bar]]

Array.wrap(“foo\nbar”) # => [“foo\nbar”]
Array(“foo\nbar”) # => [“foo\n”, “bar”], in Ruby 1.8

NOTE: Defined in active_support/core_ext/array/wrap.rb.


in_groups_of(number, fill_with = nil)

The method in_groups_of splits an array into consecutive groups of a certain size. It returns an array with the groups:

[1, 2, 3].in_groups_of(2) # => [[1, 2], [3, nil]]

or yields them in turn if a block is passed:

<% sample.in_groups_of(3) do |a, b, c| >

<=h a >
<=h b >
<=h c %>

<% end %>

The first example shows in_groups_of fills the last group with as many nil elements as needed to have the requested size. You can change this padding value using the second optional argument:

[1, 2, 3].in_groups_of(2, 0) # => [[1, 2], [3, 0]]

And you can tell the method not to fill the last group passing false:

[1, 2, 3].in_groups_of(2, false) # => [[1, 2], 3]

As a consequence false can’t be a used as a padding value.

NOTE: Defined in active_support/core_ext/array/grouping.rb.

in_groups(number, fill_with = nil)

The method in_groups splits an array into a certain number of groups. The method returns and array with the groups:

%w(1 2 3 4 5 6 7).in_groups(3)

  1. => [[“1”, “2”, “3”], [“4”, “5”, nil], [“6”, “7”, nil]]

or yields them in turn if a block is passed:

%w(1 2 3 4 5 6 7).in_groups(3) {|group| p group}
[“1”, “2”, “3”]
[“4”, “5”, nil]
[“6”, “7”, nil]

The examples above show that in_groups fills some groups with a trailing nil element as needed. A group can get at most one of these extra elements, the rightmost one if any. And the groups that have them are always the last ones.

You can change this padding value using the second optional argument:

%w(1 2 3 4 5 6 7).in_groups(3, “0”)

  1. => [[“1”, “2”, “3”], [“4”, “5”, “0”], [“6”, “7”, “0”]]

And you can tell the method not to fill the smaller groups passing false:

%w(1 2 3 4 5 6 7).in_groups(3, false)

  1. => [[“1”, “2”, “3”], [“4”, “5”], [“6”, “7”]]

As a consequence false can’t be a used as a padding value.

NOTE: Defined in active_support/core_ext/array/grouping.rb.

split(value = nil)

The method split divides an array by a separator and returns the resulting chunks.

If a block is passed the separators are those elements of the array for which the block returns true:

(-5..5).to_a.split { |i| i.multiple_of?(4) }

  1. => [[-5], [-3, -2, -1], [1, 2, 3], 5]

Otherwise, the value received as argument, which defaults to nil, is the separator:

[0, 1, -5, 1, 1, “foo”, “bar”].split(1)

  1. => [0, [-5], [], [“foo”, “bar”]]

TIP: Observe in the previous example that consecutive separators result in empty arrays.

NOTE: Defined in active_support/core_ext/array/grouping.rb.

Extensions to Hash



The method to_xml returns a string containing an XML representation of its receiver:

{"foo" => 1, “bar” => 2}.to_xml

  1. =>
  2. <?xml version=“1.0” encoding=“UTF-8”?>
  3. 1
  4. 2

To do so, the method loops over the pairs and builds nodes that depend on the values. Given a pair key, value:

  • If value is a hash there’s a recursive call with key as :root.
  • If value is an array there’s a recursive call with key as :root, and key singularized as :children.
  • If value is a callable object it must expect one or two arguments. Depending on the arity, the callable is invoked with the options hash as first argument with key as :root, and key singularized as second argument. Its return value becomes a new node.
  • If value responds to to_xml the method is invoked with key as :root.
  • Otherwise, a node with key as tag is created with a string representation of value as text node. If value is nil an attribute “nil” set to “true” is added. Unless the option :skip_types exists and is true, an attribute “type” is added as well according to the following mapping:

    “Symbol” => “symbol”,
    “Fixnum” => “integer”,
    “Bignum” => “integer”,
    “BigDecimal” => “decimal”,
    “Float” => “float”,
    “TrueClass” => “boolean”,
    “FalseClass” => “boolean”,
    “Date” => “date”,
    “DateTime” => “datetime”,
    “Time” => “datetime”

By default the root node is “hash”, but that’s configurable via the :root option.

The default XML builder is a fresh instance of Builder::XmlMarkup. You can configure your own builder with the :builder option. The method also accepts options like :dasherize and friends, they are forwarded to the builder.

NOTE: Defined in active_support/core_ext/hash/conversions.rb.


Ruby has a built-in method Hash#merge that merges two hashes:

{:a => 1, :b => 1}.merge(:a => 0, :c => 2)

  1. => {:a => 0, :b => 1, :c => 2}

Active Support defines a few more ways of merging hashes that may be convenient.

reverse_merge and reverse_merge!

In case of collision the key in the hash of the argument wins in merge. You can support option hashes with default values in a compact way with this idiom:

options = {:length => 30, :omission => “…”}.merge(options)

Active Support defines reverse_merge in case you prefer this alternative notation:

options = options.reverse_merge(:length => 30, :omission => “…”)

And a bang version reverse_merge! that performs the merge in place:

options.reverse_merge!(:length => 30, :omission => “…”)

WARNING. Take into account that reverse_merge! may change the hash in the caller, which may or may not be a good idea.

NOTE: Defined in active_support/core_ext/hash/reverse_merge.rb.


The method reverse_update is an alias for reverse_merge!, explained above.

WARNING. Note that reverse_update has no bang.

NOTE: Defined in active_support/core_ext/hash/reverse_merge.rb.

deep_merge and deep_merge!

As you can see in the previous example if a key is found in both hashes the value in the one in the argument wins.

Active Support defines Hash#deep_merge. In a deep merge, if a key is found in both hashes and their values are hashes in turn, then their merge becomes the value in the resulting hash:

{:a => {:b => 1}}.deep_merge(:a => {:c => 2})

  1. => {:a => {:b => 1, :c => 2}}

The method deep_merge! performs a deep merge in place.

NOTE: Defined in active_support/core_ext/hash/deep_merge.rb.


The method diff returns a hash that represents a diff of the receiver and the argument with the following logic:

  • Pairs key, value that exist in both hashes do not belong to the diff hash.
  • If both hashes have key, but with different values, the pair in the receiver wins.
  • The rest is just merged.

{:a => 1}.diff(:a => 1)

  1. => {}, first rule

{:a => 1}.diff(:a => 2)

  1. => {:a => 1}, second rule

{:a => 1}.diff(:b => 2)

  1. => {:a => 1, :b => 2}, third rule

{:a => 1, :b => 2, :c => 3}.diff(:b => 1, :c => 3, :d => 4)

  1. => {:a => 1, :b => 2, :d => 4}, all rules

{}.diff({}) # => {}
{:a => 1}.diff({}) # => {:a => 1}
{}.diff(:a => 1) # => {:a => 1}

An important property of this diff hash is that you can retrieve the original hash by applying diff twice:

hash.diff(hash2).diff(hash2) == hash

Diffing hashes may be useful for error messages related to expected option hashes for example.

NOTE: Defined in active_support/core_ext/hash/diff.rb.

Working with Keys

except and except!

The method except returns a hash with the keys in the argument list removed, if present:

{:a => 1, :b => 2}.except(:a) # => {:b => 2}

If the receiver responds to convert_key, the method is called on each of the arguments. This allows except to play nice with hashes with indifferent access for instance:

{:a => 1}.with_indifferent_access.except(:a) # => {}
{:a => 1}.with_indifferent_access.except(“a”) # => {}

The method except may come in handy for example when you want to protect some parameter that can’t be globally protected with attr_protected:

params[:account] = params[:account].except(:plan_id) unless admin?

There’s also the bang variant except! that removes keys in the very receiver.

NOTE: Defined in active_support/core_ext/hash/except.rb.

stringify_keys and stringify_keys!

The method stringify_keys returns a hash that has a stringified version of the keys in the receiver. It does so by sending to_s to them:

{nil => nil, 1 => 1, :a => :a}.stringify_keys

  1. => {"" => nil, “a” => :a, “1” => 1}

The result in case of collision is undefined:

{"a" => 1, :a => 2}.stringify_keys

  1. => {"a" => 2}, in my test, can’t rely on this result though

This method may be useful for example to easily accept both symbols and strings as options. For instance ActionView::Helpers::FormHelper defines:

def to_check_box_tag(options = {}, checked_value = “1”, unchecked_value = “0”)
options = options.stringify_keys
options[“type”] = “checkbox”


The second line can safely access the “type” key, and let the user to pass either :type or “type”.

There’s also the bang variant stringify_keys! that stringifies keys in the very receiver.

NOTE: Defined in active_support/core_ext/hash/keys.rb.

symbolize_keys and symbolize_keys!

The method symbolize_keys returns a hash that has a symbolized version of the keys in the receiver, where possible. It does so by sending to_sym to them:

{nil => nil, 1 => 1, “a” => "a"}.symbolize_keys

  1. => {1 => 1, nil => nil, :a => "a"}

WARNING. Note in the previous example only one key was symbolized.

The result in case of collision is undefined:

{"a" => 1, :a => 2}.symbolize_keys

  1. => {:a => 2}, in my test, can’t rely on this result though

This method may be useful for example to easily accept both symbols and strings as options. For instance ActionController::UrlRewriter defines

def rewrite_path(options)
options = options.symbolize_keys
options.update(options[:params].symbolize_keys) if options[:params]


The second line can safely access the :params key, and let the user to pass either :params or “params”.

There’s also the bang variant symbolize_keys! that symbolizes keys in the very receiver.

NOTE: Defined in active_support/core_ext/hash/keys.rb.

to_options and to_options!

The methods to_options and to_options! are respectively aliases of symbolize_keys and symbolize_keys!.

NOTE: Defined in active_support/core_ext/hash/keys.rb.


The method assert_valid_keys receives an arbitrary number of arguments, and checks whether the receiver has any key outside that white list. If it does ArgumentError is raised.

{:a => 1}.assert_valid_keys(:a) # passes
{:a => 1}.assert_valid_keys(“a”) # ArgumentError

Active Record does not accept unknown options when building associations for example. It implements that control via assert_valid_keys:

mattr_accessor :valid_keys_for_has_many_association
@@valid_keys_for_has_many_association = [
:class_name, :table_name, :foreign_key, :primary_key,
:select, :conditions, :include, :order, :group, :having, :limit, :offset,
:as, :through, :source, :source_type,
:finder_sql, :counter_sql,
:before_add, :after_add, :before_remove, :after_remove,
:extend, :readonly,
:validate, :inverse_of

def create_has_many_reflection(association_id, options, &extension)


NOTE: Defined in active_support/core_ext/hash/keys.rb.


Ruby has built-in support for taking slices out of strings and arrays. Active Support extends slicing to hashes:

{:a => 1, :b => 2, :c => 3}.slice(:a, :c)

  1. => {:c => 3, :a => 1}

{:a => 1, :b => 2, :c => 3}.slice(:b, :X)

  1. => {:b => 2} # non-existing keys are ignored

If the receiver responds to convert_key keys are normalized:

{:a => 1, :b => 2}.with_indifferent_access.slice(“a”)

  1. => {:a => 1}

NOTE. Slicing may come in handy for sanitizing option hashes with a white list of keys.

There’s also slice! which in addition to perform a slice in place returns what’s removed:

hash = {:a => 1, :b => 2}
rest = hash.slice!(:a) # => {:b => 2}
hash # => {:a => 1}

NOTE: Defined in active_support/core_ext/hash/slice.rb.

Indifferent Access

The method with_indifferent_access returns an ActiveSupport::HashWithIndifferentAccess out of its receiver:

{:a => 1}.with_indifferent_access[“a”] # => 1

NOTE: Defined in active_support/core_ext/hash/indifferent_access.rb.

Extensions to Regexp


The method multiline? says whether a regexp has the /m flag set, that is, whether the dot matches newlines.

%r{.}.multiline? # => false
%r{.}m.multiline? # => true‘.’).multiline? # => false‘.’, Regexp::MULTILINE).multiline? # => true

Rails uses this method in a single place, also in the routing code. Multiline regexps are disallowed for route requirements and this flag eases enforcing that constraint.

def assign_route_options(segments, defaults, requirements)

if requirement.multiline?
raise ArgumentError, “Regexp multiline option not allowed in routing requirements: #{requirement.inspect}”


NOTE: Defined in active_support/core_ext/regexp.rb.

Extensions to Range


Active Support extends the method Range#to_s so that it understands an optional format argument. As of this writing the only supported non-default format is :db:


  1. => “2009-10-25..2009-10-26”


  1. => “BETWEEN ‘2009-10-25’ AND ‘2009-10-26’”

As the example depicts, the :db format generates a BETWEEN SQL clause. That is used by Active Record in its support for range values in conditions.

NOTE: Defined in active_support/core_ext/range/conversions.rb.


Active Support extends the method Range#step so that it can be invoked without a block:

(1..10).step(2) # => [1, 3, 5, 7, 9]

As the example shows, in that case the method returns and array with the corresponding elements.

NOTE: Defined in active_support/core_ext/range/blockless_step.rb.


The method Range#include? says whether some value falls between the ends of a given instance:

(2..3).include?(Math::E) # => true

Active Support extends this method so that the argument may be another range in turn. In that case we test whether the ends of the argument range belong to the receiver themselves:

(1..10).include?(3..7) # => true
(1..10).include?(0..7) # => false
(1..10).include?(3..11) # => false
(1…9).include?(3..9) # => false

WARNING: The original Range#include? is still the one aliased to Range#===.

NOTE: Defined in active_support/core_ext/range/include_range.rb.


The method Range#overlaps? says whether any two given ranges have non-void intersection:

(1..10).overlaps?(7..11) # => true
(1..10).overlaps?(0..7) # => true
(1..10).overlaps?(11..27) # => false

NOTE: Defined in active_support/core_ext/range/overlaps.rb.

Extensions to Proc


As you surely know Ruby has an UnboundMethod class whose instances are methods that belong to the limbo of methods without a self. The method Module#instance_method returns an unbound method for example:

Hash.instance_method(:delete) # => #<UnboundMethod: Hash#delete>

An unbound method is not callable as is, you need to bind it first to an object with bind:

clear = Hash.instance_method(:clear)
clear.bind({:a => 1}).call # => {}

Active Support defines Proc#bind with an analogous purpose: { size }.bind([]).call # => 0

As you see that’s callable and bound to the argument, the return value is indeed a Method.

NOTE: To do so Proc#bind actually creates a method under the hood. If you ever see a method with a weird name like __bind_1256598120_237302 in a stack trace you know now where it comes from.

Action Pack uses this trick in rescue_from for example, which accepts the name of a method and also a proc as callbacks for a given rescued exception. It has to call them in either case, so a bound method is returned by handler_for_rescue, thus simplifying the code in the caller:

def handler_for_rescue(exception)
, rescuer = Array(rescue_handlers).reverse.detect do |klassname, handler|


case rescuer when Symbol method(rescuer) when Proc rescuer.bind(self) end


NOTE: Defined in active_support/core_ext/proc.rb.

Extensions to Date


NOTE: All the following methods are defined in active_support/core_ext/date/calculations.rb.

INFO: The following calculation methods have edge cases in October 1582, since days 5..14 just do not exist. This guide does not document their behaviour around those days for brevity, but it is enough to say that they do what you would expect. That is,, 10, 4).tomorrow returns, 10, 15) and so on. Please check test/core_ext/date_ext_test.rb in the Active Support test suite for expected behaviour.


Active Support defines Date.current to be today in the current time zone. That’s like, except that it honors the user time zone, if defined. It also defines Date.yesterday and Date.tomorrow, and the instance predicates past?, today?, and future?, all of them relative to Date.current.

Named dates
prev_year, next_year

In Ruby 1.9 prev_year and next_year return a date with the same day/month in the last or next year:

d =, 5, 8) # => Sat, 08 May 2010
d.prev_year # => Fri, 08 May 2009
d.next_year # => Sun, 08 May 2011

If date is the 29th of February of a leap year, you obtain the 28th:

d =, 2, 29) # => Tue, 29 Feb 2000
d.prev_year # => Sun, 28 Feb 1999
d.next_year # => Wed, 28 Feb 2001

Active Support defines these methods as well for Ruby 1.8.

prev_month, next_month

In Ruby 1.9 prev_month and next_month return the date with the same day in the last or next month:

d =, 5, 8) # => Sat, 08 May 2010
d.prev_month # => Thu, 08 Apr 2010
d.next_month # => Tue, 08 Jun 2010

If such a day does not exist, the last day of the corresponding month is returned:, 5, 31).prev_month # => Sun, 30 Apr 2000, 3, 31).prev_month # => Tue, 29 Feb 2000, 5, 31).next_month # => Fri, 30 Jun 2000, 1, 31).next_month # => Tue, 29 Feb 2000

Active Support defines these methods as well for Ruby 1.8.

beginning_of_week, end_of_week

The methods beginning_of_week and end_of_week return the dates for the beginning and end of week, assuming weeks start on Monday:

d =, 5, 8) # => Sat, 08 May 2010
d.beginning_of_week # => Mon, 03 May 2010
d.end_of_week # => Sun, 09 May 2010

beginning_of_week is aliased to monday and at_beginning_of_week. end_of_week is aliased to sunday and at_end_of_week.


next_week receives a symbol with a day name in English (in lowercase, default is :monday) and it returns the date corresponding to that day in the next week:

d =, 5, 9) # => Sun, 09 May 2010
d.next_week # => Mon, 10 May 2010
d.next_week(:saturday) # => Sat, 15 May 2010

beginning_of_month, end_of_month

The methods beginning_of_month and end_of_month return the dates for the beginning and end of the month:

d =, 5, 9) # => Sun, 09 May 2010
d.beginning_of_month # => Sat, 01 May 2010
d.end_of_month # => Mon, 31 May 2010

beginning_of_month is aliased to at_beginning_of_month, and end_of_month is aliased to at_end_of_month.

beginning_of_quarter, end_of_quarter

The methods beginning_of_quarter and end_of_quarter return the dates for the beginning and end of the quarter of the receiver’s calendar year:

d =, 5, 9) # => Sun, 09 May 2010
d.beginning_of_quarter # => Thu, 01 Apr 2010
d.end_of_quarter # => Wed, 30 Jun 2010

beginning_of_quarter is aliased to at_beginning_of_quarter, and end_of_quarter is aliased to at_end_of_quarter.

beginning_of_year, end_of_year

The methods beginning_of_year and end_of_year return the dates for the beginning and end of the year:

d =, 5, 9) # => Sun, 09 May 2010
d.beginning_of_year # => Fri, 01 Jan 2010
d.end_of_year # => Fri, 31 Dec 2010

beginning_of_year is aliased to at_beginning_of_year, and end_of_year is aliased to at_end_of_year.

Other Date Computations
years_ago, years_since

The method years_ago receives a number of years and returns the same date those many years ago:

date =, 6, 7)
date.years_ago(10) # => Wed, 07 Jun 2000

years_since moves forward in time:

date =, 6, 7)
date.years_since(10) # => Sun, 07 Jun 2020

If such a day does not exist, the last day of the corresponding month is returned:, 2, 29).years_ago(3) # => Sat, 28 Feb 2009, 2, 29).years_since(3) # => Sat, 28 Feb 2015

months_ago, months_since

The methods months_ago and months_since work analogously for months:, 4, 30).months_ago(2) # => Sun, 28 Feb 2010, 4, 30).months_since(2) # => Wed, 30 Jun 2010

If such a day does not exist, the last day of the corresponding month is returned:, 4, 30).months_ago(2) # => Sun, 28 Feb 2010, 12, 31).months_since(2) # => Sun, 28 Feb 2010


The most generic way to jump to other days is advance. This method receives a hash with keys :years, :months, :weeks, :days, and returns a date advanced as much as the present keys indicate:

date =, 6, 6)
date.advance(:years => 1, :weeks => 2) # => Mon, 20 Jun 2011
date.advance(:months => 2, :days => -2) # => Wed, 04 Aug 2010

Note in the previous example that increments may be negative.

To perform the computation the method first increments years, then months, then weeks, and finally days. This order is important towards the end of months. Say for example we are at the end of February of 2010, and we want to move one month and one day forward.

The method advance advances first one month, and the one day, the result is:, 2, 28).advance(:months => 1, :day => 1)

  1. => Sun, 28 Mar 2010

While if it did it the other way around the result would be different:, 2, 28).advance(:days => 1).advance(:months => 1)

  1. => Thu, 01 Apr 2010
Changing Components

The method change allows you to get a new date which is the same as the receiver except for the given year, month, or day:, 12, 23).change(:year => 2011, :month => 11)

  1. => Wed, 23 Nov 2011

This method is not tolerant to non-existing dates, if the change is invalid ArgumentError is raised:, 1, 31).change(:month => 2)

  1. => ArgumentError: invalid date

INFO: The following methods return a Time object if possible, otherwise a DateTime. If set, they honor the user time zone.

beginning_of_day, end_of_day

The method beginning_of_day returns a timestamp at the beginning of the day (00:00:00):

date =, 6, 7)
date.beginning_of_day # => Sun Jun 07 00:00:00 +0200 2010

The method end_of_day returns a timestamp at the end of the day (23:59:59):

date =, 6, 7)
date.end_of_day # => Sun Jun 06 23:59:59 +0200 2010

beginning_of_day is aliased to at_beginning_of_day, midnight, at_midnight.

ago, since

The method ago receives a number of seconds as argument and returns a timestamp those many seconds ago from midnight:

date = Date.current # => Fri, 11 Jun 2010
date.ago(1) # => Thu, 10 Jun 2010 23:59:59 EDT -04:00

Similarly, since moves forward:

date = Date.current # => Fri, 11 Jun 2010
date.since(1) # => Fri, 11 Jun 2010 00:00:01 EDT -04:00

Other Time Computations


Extensions to DateTime

NOTE: All the following methods are defined in active_support/core_ext/date_time/calculations.rb.

WARNING: DateTime is not aware of DST rules and so some of these methods have edge cases when a DST change is going on. For example seconds_since_midnight might not return the real amount in such a day.


The class DateTime is a subclass of Date so by loading active_support/core_ext/date/calculations.rb you inherit these methods and their aliases, except that they will always return datetimes:


The following methods are reimplemented so you do not need to load active_support/core_ext/date/calculations.rb for these ones:


On the other hand, advance and change are also defined and support more options, they are documented below.

Named Datetimes

Active Support defines DateTime.current to be like, except that it honors the user time zone, if defined. It also defines instance predicates past?, and future? relative to DateTime.current.

Other Extensions

The method seconds_since_midnight returns the number of seconds since midnight:

now = DateTime.current # => Mon, 07 Jun 2010 20:26:36 +0000
now.seconds_since_midnight # => 73596


The method utc gives you the same datetime in the receiver expressed in UTC.

now = DateTime.current # => Mon, 07 Jun 2010 19:27:52 -0400
now.utc # => Mon, 07 Jun 2010 23:27:52 +0000

This method is also aliased as getutc.


The predicate utc? says whether the receiver has UTC as its time zone:

now = # => Mon, 07 Jun 2010 19:30:47 -0400
now.utc? # => false
now.utc.utc? # => true

Changing Components

The method change allows you to get a new datetime which is the same as the receiver except for the given options, which may include :year, :month, :day, :hour, :min, :sec, :offset, :start:

now = DateTime.current

  1. => Tue, 08 Jun 2010 01:56:22 +0000
    now.change(:year => 2011, :offset => Rational(-6, 24))
  2. => Wed, 08 Jun 2011 01:56:22 -0600

If hours are zeroed, then minutes and seconds are too (unless they have given values):

now.change(:hour => 0)

  1. => Tue, 08 Jun 2010 00:00:00 +0000

Similarly, if minutes are zeroed, then seconds are too (unless it has given a value):

now.change(:min => 0)

  1. => Tue, 08 Jun 2010 01:00:00 +0000

This method is not tolerant to non-existing dates, if the change is invalid ArgumentError is raised:

DateTime.current.change(:month => 2, :day => 30)

  1. => ArgumentError: invalid date


Extensions to Time

Extensions to Process

Extensions to File


With the class method File.atomic_write you can write to a file in a way that will prevent any reader from seeing half-written content.

The name of the file is passed as an argument, and the method yields a file handle opened for writing. Once the block is done atomic_write closes the file handle and completes its job.

For example, Action Pack uses this method to write asset cache files like all.css:

File.atomic_write(joined_asset_path) do |cache|

To accomplish this atomic_write creates a temporary file. That’s the file the code in the block actually writes to. On completion, the temporary file is renamed, which is an atomic operation on POSIX systems. If the target file exists atomic_write overwrites it and keeps owners and permissions.

WARNING. Note you can’t append with atomic_write.

The auxiliary file is written in a standard directory for temporary files, but you can pass a directory of your choice as second argument.

NOTE: Defined in active_support/core_ext/file/atomic.rb.

Extensions to NameError

Active Support adds missing_name? to NameError, which tests whether the exception was raised because of the name passed as argument.

The name may be given as a symbol or string. A symbol is tested against the bare constant name, a string is against the fully-qualified constant name.

TIP: A symbol can represent a fully-qualified constant name as in :“ActiveRecord::Base”, so the behaviour for symbols is defined for convenience, not because it has to be that way technically.

For example, when an action of PostsController is called Rails tries optimistically to use PostsHelper. It is OK that the helper module does not exist, so if an exception for that constant name is raised it should be silenced. But it could be the case that posts_helper.rb raises a NameError due to an actual unknown constant. That should be reraised. The method missing_name? provides a way to distinguish both cases:

def default_helper_module!
module_name = name.sub(/Controller$/, ’’)
module_path = module_name.underscore
helper module_path
rescue MissingSourceFile => e
raise e unless e.is_missing? “#{module_path}_helper”
rescue NameError => e
raise e unless e.missing_name? “#{module_name}Helper”

NOTE: Defined in active_support/core_ext/name_error.rb.

Extensions to LoadError

Active Support adds is_missing? to LoadError, and also assigns that class to the constant MissingSourceFile for backwards compatibility.

Given a path name is_missing? tests whether the exception was raised due to that particular file (except perhaps for the “.rb” extension).

For example, when an action of PostsController is called Rails tries to load posts_helper.rb, but that file may not exist. That’s fine, the helper module is not mandatory so Rails silences a load error. But it could be the case that the helper module does exist and in turn requires another library that is missing. In that case Rails must reraise the exception. The method is_missing? provides a way to distinguish both cases:

def default_helper_module!
module_name = name.sub(/Controller$/, ’’)
module_path = module_name.underscore
helper module_path
rescue MissingSourceFile => e
raise e unless e.is_missing? “helpers/#{module_path}_helper”
rescue NameError => e
raise e unless e.missing_name? “#{module_name}Helper”

NOTE: Defined in active_support/core_ext/load_error.rb.


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