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Active Support Overview

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 modules and classes provided by Rails.
  • The rest of fundamental libraries available in Rails.


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?


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.


The method returning yields its argument to a block and returns it. You tipically 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)

See also Object#tap.


Object#tap exists in Ruby 1.8.7 and 1.9, and it is defined by Active Support for previous versions. This method yields its receiver to a block and returns it.

For example, the following class method from ActionDispatch::TestResponse creates, initializes, and returns a new test response using tap:

def self.from_response(response)
new.tap do |resp|
resp.status = response.status
resp.headers = response.headers
resp.body = response.body

See also Object#returning.


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 synonim 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(’ ’)))


The method metaclass returns the singleton class on any object:

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

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)


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.


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 redifinition of to_param because when a request like that comes in “357-john-smith” is the value of params[:id].


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 assigments 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”


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.

Instance Variables

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


The method instance_variable_defined? exists in Ruby 1.8.6 and later, and it is defined for previous versions anyway:

class C
def initialize
@a = 1

def m @b = 2 end


c =

c.instance_variable_defined?(“@a”) # => true
c.instance_variable_defined?(:@a) # => true
c.instance_variable_defined?(“a”) # => NameError: `a’ is not allowed as an instance variable name

c.instance_variable_defined?(“@b”) # => false
c.instance_variable_defined?(“@b”) # => true


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 unespecified, and it indeed depends on the version of the interpreter.


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}

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.

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

Extensions to Module

Extensions to Class

Class Attribute Accessors

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

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 accesors 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.

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.


The subclasses method returns the names of all subclasses of a given class as an array of strings. That comprises not only direct subclasses, but all descendants down the hierarchy:

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.

Class Removal

Roughly speaking, the remove_class method removes the class objects passed as arguments:

Class.remove_class(Hash, Dir) # => [Hash, Dir]
Hash # => NameError: uninitialized constant Hash
Dir # => NameError: uninitialized constant Dir

More specifically, remove_class attempts to remove constants with the same name as the passed class objects from their parent modules. So technically this method does not guarantee the class objects themselves are not still valid and alive somewhere after the method call:

module M
class A; end
class B < A; end

A2 = M::A

M::A.object_id # => 13053950

M::B.superclass.object_id # => 13053950 (same object as before) # => “M::A” (name is hard-coded in object)

WARNING: Removing fundamental classes like String can result in really funky behaviour.

The method remove_subclasses provides a shortcut for removing all descendants of a given class, where “removing” has the meaning explained above:

class A; end
class B1 < A; end
class B2 < A; end
class C < A; end

A.subclasses # => [“C”, “B2”, “B1”]
A.subclasses # => []
C # => NameError: uninitialized constant C

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

Extensions to Symbol


The method to_proc turns a symbol into a Proc object so that for example

emails = {|u|}

can be written as

emails =

TIP: If the method that receives the Proc yields more than one value to it the rest are considered to be arguments of the method call.

Symbols from Ruby 1.8.7 on respond to to_proc, and Active Support defines it for previous versions.

Extensions to String

Extensions to Numeric

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

WARNING: Due the way it is implemented the argument must be nonzero, otherwise ZeroDivisionError is raised.

even? and odd?

Integers in Ruby 1.8.7 and above respond to even? and odd?, Active Support defines them for older versions:

-1.even? # => false
-1.odd? # => true
0.even? # => true
0.odd? # => false
2.even? # => true
2.odd? # => false


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

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

Extensions to Float

Extensions to BigDecimal

Extensions to Enumerable

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) # => nil

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

You can pick a random element with rand:

shape_type = [Circle, Square, Triangle].rand


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.

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.

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”]]

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

Extensions to Hash

Extensions to Range

Extensions to Proc

Extensions to Date

Extensions to DateTime

Extensions to Time

Extensions to Process

Extensions to Pathname

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. 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.

Extensions to Exception

Extensions to NameError

Extensions to LoadError

Extensions to CGI

Extensions to Benchmark


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