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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.

endprologue.

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
end

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?

duplicable?

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.

returning

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)
end
end

See also Object#tap.

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
end
end

See also Object#returning.

try

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(’ ’)))
end
end

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(’ ’)))
end
end

metaclass

The method metaclass returns the singleton class on any object:

String.metaclass # => #
String.new.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, Time.now
object.class_eval do
method_name = “__bind_#{time.to_i}_#{time.usec}”
define_method(method_name, &block)
method = instance_method(method_name)
remove_method(method_name)
method
end.bind(object)
end
end

acts_like?(duck)

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?
end

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

some_klass.acts_like?(:string)

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

to_param

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
“#{id}-#{name.parameterize}”
end
end

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].

to_query

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
“#{id}-#{name.parameterize}”
end
end

we get:

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

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

account.to_query(‘company[name]’)

  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”

with_options

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
end

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
end
end

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 => user.name
end

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.

instance_variable_defined?

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
end

def m @b = 2 end

end

c = C.new

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

instance_variable_names

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
end

C.new(0, 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.

instance_values

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
end

C.new(0, 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
end

def protected_instance_variables %w(@z) end

end

a = C.new(0, 1, 2)
b = C.new(3, 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
    end

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
    end

Extensions to Module

Aliasing

alias_method_chain

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[*params.map {|k, v| [k, v.to_s]}.flatten]
    original_process(action, params, session, flash, http_method)
    end
    end

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[*params.map {|k, v| [k, v.to_s]}.flatten]
process_without_stringified_params(action, params, session, flash, http_method)
end
alias_method :process_without_stringified_params, :process
alias_method :process, :process_with_stringified_params
end

The method alias_method_chain provides a shortcut for that pattern:

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

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.

alias_attribute

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
    end

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
    end

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

module ActionController
class Base
cattr_accessor :logger
end
end

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
    end
    end

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

  1. FIXME: REVISE/SIMPLIFY THIS COMMENT.
  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
    end
    end

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:

class_inheritable_array
class_inheritable_hash

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
end
end

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
end
end

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.

Subclasses

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
end

module N
class C < M::B1; end
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
end

A2 = M::A

M::A.object_id # => 13053950
Class.remove_class(M::A)

M::B.superclass.object_id # => 13053950 (same object as before)
A2.name # => “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.remove_subclasses
A.subclasses # => []
C # => NameError: uninitialized constant C

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

Extensions to Symbol

to_proc

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

emails = users.map {|u| u.email}

can be written as

emails = users.map(&:email)

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

bytesize

Ruby 1.9 introduces String#bytesize to obtain the length of a string in bytes. Ruby 1.8.7 defines this method as an alias for String#size for forward compatibility, and Active Support does so for previous versions.

squish

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!.

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.

start_with? and end_width?

Ruby 1.8.7 and up define the predicates String#start_with? and String#end_with?:

“foo”.start_with?(“f”) # => true
“foo”.start_with?(“g”) # => false
“foo”.start_with?("") # => true

“foo”.end_with?(“o”) # => true
“foo”.end_with?(“p”) # => false
“foo”.end_with?("") # => true

If strings do not respond to those methods Active Support emulates them, and also defines their 3rd person aliases:

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

in case you feel more comfortable spelling them that way.

WARNING. Active Support invokes to_s on the argument, but Ruby does not. Since Active Support defines these methods only if strings do not respond to them, this corner of their behaviour depends on the interpreter that runs a given Rails application. You change the interpreter, and start_with?(1) may change its return value. In consequence, it’s more portable not to rely on that and pass always strings.

each_char

Ruby 1.8.7 and up define the iterator String#each_char that understands UTF8 and yields strings with a single character each, so they have length 1 but may be multibyte. Active Support defines that method for previous versions of Ruby:

“\xE6\x97\xA5\xE6\x9C\xAC\xE8\xAA\x9E”.each_char {|c| print c} # => 日本語

Access

at(position)

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

from(position)

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

to(position)

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”

first(limit = 1)

The call str.first(n) is equivalent to str.to(n-1) if n > 0, and returns an empty string for n == 0.

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.

Extensions to Numeric

Extensions to Integer

multiple_of?

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

ordinalize

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

group_by

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|
entry.surname.at(0).upcase
end

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

sum

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) }
end

each_with_object

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.

index_by

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:

invoices.index_by(&:number)

  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.

many?

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]}

none?

The method none? is the negation of any?. It yields elements to a block and returns true if none of them matches:

success = responses.none? {|r| r.status / 100 == 5}

Ruby 1.8.7 and later already have none?, Active Support defines it for previous versions.

Extensions to Array

Accessing

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

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!

end

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.

Conversions

to_sentence

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.

to_formatted_s

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.

to_xml

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

Wrapping

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

Grouping

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

Conversions

to_xml

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:

    XML_TYPE_NAMES = {
    “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.

Merging

Ruby has a builtin 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.

reverse_update

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

WARNING. Note that reverse_update has no bang.

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.

Diffing

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:

hash1.diff(hash2).diff(hash2) == hash1

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

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?
@account.update_attributes(params[:account])

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

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”

end

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.

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]

end

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.

to_options and to_options!

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

assert_valid_keys

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,
:dependent,
:select, :conditions, :include, :order, :group, :having, :limit, :offset,
:as, :through, :source, :source_type,
:uniq,
: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)
options.assert_valid_keys(valid_keys_for_has_many_association)

end

Slicing

Ruby has builtin 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}

Indifferent Access

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

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

Extensions to Regexp

number_of_captures

The method number_of_captures returns the number of capturing groups in a given regexp:

%r{}.number_of_captures # => 0
%r{.(.).}.number_of_captures # => 1
%r{\A((#)(\w+|\s+))\z}.number_of_captures # => 3

Routing code for example uses that method to generate path recognizers:

def recognition_extraction
next_capture = 1
extraction = segments.collect do |segment|
x = segment.match_extraction(next_capture)
next_capture += segment.number_of_captures
x
end
extraction.compact
end

multiline?

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

Regexp.new(‘.’).multiline? # => false
Regexp.new(‘.’, 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}”
end

end

Extensions to Range

Extensions to Proc

Extensions to Date

Extensions to DateTime

Extensions to Time

Extensions to Process

Extensions to Pathname

Extensions to File

atomic_write

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|
cache.write(join_asset_file_contents(asset_paths))
end

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

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”
end

Extensions to LoadError

Rails hijacks LoadError.new to return a MissingSourceFile exception:

$ ruby e ‘require “nonexistent”’
…: no such file to load -
nonexistent (LoadError)

$ script/runner ‘require “nonexistent”’
…: no such file to load — nonexistent (MissingSourceFile)

The class MissingSourceFile is a subclass of LoadError, so any code that rescues LoadError as usual still works as expected. Point is these exception objects respond to is_missing?, which given a path name 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, but it 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? “#{module_path}_helper”
rescue NameError => e
raise e unless e.missing_name? “#{module_name}Helper”
end

Extensions to CGI

Extensions to Benchmark

Changelog

Lighthouse ticket

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