class Module

Allows you to make aliases for attributes, which includes getter, setter, and query methods.

class Content < ActiveRecord::Base
  # has a title attribute
end

class Email < Content
  alias_attribute :subject, :title
end

e = Email.find(1)
e.title    # => "Superstars"
e.subject  # => "Superstars"
e.subject? # => true
e.subject = "Megastars"
e.title    # => "Megastars"

# File activesupport/lib/active_support/core_ext/module/aliasing.rb, line 62
  def alias_attribute(new_name, old_name)
    module_eval "      def #{new_name}; self.#{old_name}; end          # def subject; self.title; end
      def #{new_name}?; self.#{old_name}?; end        # def subject?; self.title?; end
      def #{new_name}=(v); self.#{old_name} = v; end  # def subject=(v); self.title = v; end
", __FILE__, __LINE__ + 1
  end

Encapsulates the common pattern of:

alias_method :foo_without_feature, :foo
alias_method :foo, :foo_with_feature

With this, you simply do:

alias_method_chain :foo, :feature

And both aliases are set up for you.

Query and bang methods (foo?, foo!) keep the same punctuation:

alias_method_chain :foo?, :feature

is equivalent to

alias_method :foo_without_feature?, :foo?
alias_method :foo?, :foo_with_feature?

so you can safely chain foo, foo?, and foo! with the same feature.

# File activesupport/lib/active_support/core_ext/module/aliasing.rb, line 23
def alias_method_chain(target, feature)
  # Strip out punctuation on predicates or bang methods since
  # e.g. target?_without_feature is not a valid method name.
  aliased_target, punctuation = target.to_s.sub(/([?!=])$/, ''), $1
  yield(aliased_target, punctuation) if block_given?

  with_method = "#{aliased_target}_with_#{feature}#{punctuation}"
  without_method = "#{aliased_target}_without_#{feature}#{punctuation}"

  alias_method without_method, target
  alias_method target, with_method

  case
  when public_method_defined?(without_method)
    public target
  when protected_method_defined?(without_method)
    protected target
  when private_method_defined?(without_method)
    private target
  end
end

A module may or may not have a name.

module M; end
M.name # => "M"

m = Module.new
m.name # => nil

A module gets a name when it is first assigned to a constant. Either via the module or class keyword or by an explicit assignment:

m = Module.new # creates an anonymous module
M = m          # => m gets a name here as a side-effect
m.name         # => "M"

# File activesupport/lib/active_support/core_ext/module/anonymous.rb, line 16
def anonymous?
  name.nil?
end

Declares an attribute reader and writer backed by an internally-named instance variable.

# File activesupport/lib/active_support/core_ext/module/attr_internal.rb, line 14
def attr_internal_accessor(*attrs)
  attr_internal_reader(*attrs)
  attr_internal_writer(*attrs)
end

Declares an attribute reader backed by an internally-named instance variable.

# File activesupport/lib/active_support/core_ext/module/attr_internal.rb, line 3
def attr_internal_reader(*attrs)
  attrs.each {|attr_name| attr_internal_define(attr_name, :reader)}
end

Declares an attribute writer backed by an internally-named instance variable.

# File activesupport/lib/active_support/core_ext/module/attr_internal.rb, line 8
def attr_internal_writer(*attrs)
  attrs.each {|attr_name| attr_internal_define(attr_name, :writer)}
end

Provides a delegate class method to easily expose contained objects' public methods as your own.

Options

• :to - Specifies the target object

• :prefix - Prefixes the new method with the target name or a custom prefix

• :allow_nil - if set to true, prevents a NoMethodError to be raised

The macro receives one or more method names (specified as symbols or strings) and the name of the target object via the :to option (also a symbol or string).

Delegation is particularly useful with Active Record associations:

class Greeter < ActiveRecord::Base
  def hello
    'hello'
  end

  def goodbye
    'goodbye'
  end
end

class Foo < ActiveRecord::Base
  belongs_to :greeter
  delegate :hello, to: :greeter
end

Foo.new.hello   # => "hello"
Foo.new.goodbye # => NoMethodError: undefined method `goodbye' for #<Foo:0x1af30c>

Multiple delegates to the same target are allowed:

class Foo < ActiveRecord::Base
  belongs_to :greeter
  delegate :hello, :goodbye, to: :greeter
end

Foo.new.goodbye # => "goodbye"

Methods can be delegated to instance variables, class variables, or constants by providing them as a symbols:

class Foo
  CONSTANT_ARRAY = [0,1,2,3]
  @@class_array  = [4,5,6,7]

  def initialize
    @instance_array = [8,9,10,11]
  end
  delegate :sum, to: :CONSTANT_ARRAY
  delegate :min, to: :@@class_array
  delegate :max, to: :@instance_array
end

Foo.new.sum # => 6
Foo.new.min # => 4
Foo.new.max # => 11

It's also possible to delegate a method to the class by using :class:

class Foo
  def self.hello
    "world"
  end

  delegate :hello, to: :class
end

Foo.new.hello # => "world"

Delegates can optionally be prefixed using the :prefix option. If the value is true, the delegate methods are prefixed with the name of the object being delegated to.

Person = Struct.new(:name, :address)

class Invoice < Struct.new(:client)
  delegate :name, :address, to: :client, prefix: true
end

john_doe = Person.new('John Doe', 'Vimmersvej 13')
invoice = Invoice.new(john_doe)
invoice.client_name    # => "John Doe"
invoice.client_address # => "Vimmersvej 13"

It is also possible to supply a custom prefix.

class Invoice < Struct.new(:client)
  delegate :name, :address, to: :client, prefix: :customer
end

invoice = Invoice.new(john_doe)
invoice.customer_name    # => 'John Doe'
invoice.customer_address # => 'Vimmersvej 13'

If the target is nil and does not respond to the delegated method a NoMethodError is raised, as with any other value. Sometimes, however, it makes sense to be robust to that situation and that is the purpose of the :allow_nil option: If the target is not nil, or it is and responds to the method, everything works as usual. But if it is nil and does not respond to the delegated method, nil is returned.

class User < ActiveRecord::Base
  has_one :profile
  delegate :age, to: :profile
end

User.new.age # raises NoMethodError: undefined method `age'

But if not having a profile yet is fine and should not be an error condition:

class User < ActiveRecord::Base
  has_one :profile
  delegate :age, to: :profile, allow_nil: true
end

User.new.age # nil

Note that if the target is not nil then the call is attempted regardless of the :allow_nil option, and thus an exception is still raised if said object does not respond to the method:

class Foo
  def initialize(bar)
    @bar = bar
  end

  delegate :name, to: :@bar, allow_nil: true
end

Foo.new("Bar").name # raises NoMethodError: undefined method `name'

The target method must be public, otherwise it will raise NoMethodError.

# File activesupport/lib/active_support/core_ext/module/delegation.rb, line 143
def delegate(*methods)
  options = methods.pop
  unless options.is_a?(Hash) && to = options[:to]
    raise ArgumentError, 'Delegation needs a target. Supply an options hash with a :to key as the last argument (e.g. delegate :hello, to: :greeter).'
  end

  prefix, allow_nil = options.values_at(:prefix, :allow_nil)

  if prefix == true && to =~ /^[^a-z_]/
    raise ArgumentError, 'Can only automatically set the delegation prefix when delegating to a method.'
  end

  method_prefix =        if prefix
      "#{prefix == true ? to : prefix}_"
    else
      ''
    end

  file, line = caller.first.split(':', 2)
  line = line.to_i

  to = to.to_s
  to = 'self.class' if to == 'class'

  methods.each do |method|
    # Attribute writer methods only accept one argument. Makes sure []=
    # methods still accept two arguments.
    definition = (method =~ /[^\]]=$/) ? 'arg' : '*args, &block'

    # The following generated methods call the target exactly once, storing
    # the returned value in a dummy variable.
    #
    # Reason is twofold: On one hand doing less calls is in general better.
    # On the other hand it could be that the target has side-effects,
    # whereas conceptually, from the user point of view, the delegator should
    # be doing one call.
    if allow_nil
      method_def = [
        "def #{method_prefix}#{method}(#{definition})",  # def customer_name(*args, &block)
        "_ = #{to}",                                     #   _ = client
        "if !_.nil? || nil.respond_to?(:#{method})",     #   if !_.nil? || nil.respond_to?(:name)
        "  _.#{method}(#{definition})",                  #     _.name(*args, &block)
        "end",                                           #   end
      "end"                                              # end
      ].join ';'
    else
      exception = %Q(raise DelegationError, "#{self}##{method_prefix}#{method} delegated to #{to}.#{method}, but #{to} is nil: \#{self.inspect}")

      method_def = [
        "def #{method_prefix}#{method}(#{definition})",  # def customer_name(*args, &block)
        " _ = #{to}",                                    #   _ = client
        "  _.#{method}(#{definition})",                  #   _.name(*args, &block)
        "rescue NoMethodError => e",                     # rescue NoMethodError => e
        "  if _.nil? && e.name == :#{method}",           #   if _.nil? && e.name == :name
        "    #{exception}",                              #     # add helpful message to the exception
        "  else",                                        #   else
        "    raise",                                     #     raise
        "  end",                                         #   end
        "end"                                            # end
      ].join ';'
    end

    module_eval(method_def, file, line)
  end
end

deprecate :foo
deprecate bar: 'message'
deprecate :foo, :bar, baz: 'warning!', qux: 'gone!'

You can also use custom deprecator instance:

deprecate :foo, deprecator: MyLib::Deprecator.new
deprecate :foo, bar: "warning!", deprecator: MyLib::Deprecator.new

Custom deprecators must respond to deprecation_warning(deprecated_method_name, message, caller_backtrace) method where you can implement your custom warning behavior.

class MyLib::Deprecator
  def deprecation_warning(deprecated_method_name, message, caller_backtrace = nil)
     message = "#{deprecated_method_name} is deprecated and will be removed from MyLibrary | #{message}"
     Kernel.warn message
  end
end

# File activesupport/lib/active_support/core_ext/module/deprecation.rb, line 20
def deprecate(*method_names)
  ActiveSupport::Deprecation.deprecate_methods(self, *method_names)
end

Defines both class and instance accessors for class attributes.

module HairColors
  mattr_accessor :hair_colors
end

class Person
  include HairColors
end

Person.hair_colors = [:brown, :black, :blonde, :red]
Person.hair_colors     # => [:brown, :black, :blonde, :red]
Person.new.hair_colors # => [:brown, :black, :blonde, :red]

If a subclass changes the value then that would also change the value for parent class. Similarly if parent class changes the value then that would change the value of subclasses too.

class Male < Person
end

Male.hair_colors << :blue
Person.hair_colors # => [:brown, :black, :blonde, :red, :blue]

To opt out of the instance writer method, pass instance_writer: false. To opt out of the instance reader method, pass instance_reader: false.

module HairColors
  mattr_accessor :hair_colors, instance_writer: false, instance_reader: false
end

class Person
  include HairColors
end

Person.new.hair_colors = [:brown]  # => NoMethodError
Person.new.hair_colors             # => NoMethodError

Or pass instance_accessor: false, to opt out both instance methods.

module HairColors
  mattr_accessor :hair_colors, instance_accessor: false
end

class Person
  include HairColors
end

Person.new.hair_colors = [:brown]  # => NoMethodError
Person.new.hair_colors             # => NoMethodError

Also you can pass a block to set up the attribute with a default value.

module HairColors
  mattr_accessor :hair_colors do
    [:brown, :black, :blonde, :red]
  end
end

class Person
  include HairColors
end

Person.class_variable_get("@@hair_colors") #=> [:brown, :black, :blonde, :red]

# File activesupport/lib/active_support/core_ext/module/attribute_accessors.rb, line 207
def mattr_accessor(*syms, &blk)
  mattr_reader(*syms, &blk)
  mattr_writer(*syms, &blk)
end

Defines a class attribute and creates a class and instance reader methods. The underlying the class variable is set to nil, if it is not previously defined.

module HairColors
  mattr_reader :hair_colors
end

HairColors.hair_colors # => nil
HairColors.class_variable_set("@@hair_colors", [:brown, :black])
HairColors.hair_colors # => [:brown, :black]

The attribute name must be a valid method name in Ruby.

module Foo
  mattr_reader :"1_Badname "
end
# => NameError: invalid attribute name

If you want to opt out the creation on the instance reader method, pass instance_reader: false or instance_accessor: false.

module HairColors
  mattr_writer :hair_colors, instance_reader: false
end

class Person
  include HairColors
end

Person.new.hair_colors # => NoMethodError

Also, you can pass a block to set up the attribute with a default value.

module HairColors
  cattr_reader :hair_colors do
    [:brown, :black, :blonde, :red]
  end
end

class Person
  include HairColors
end

Person.hair_colors # => [:brown, :black, :blonde, :red]

# File activesupport/lib/active_support/core_ext/module/attribute_accessors.rb, line 53
  def mattr_reader(*syms)
    options = syms.extract_options!
    syms.each do |sym|
      raise NameError.new("invalid attribute name: #{sym}") unless sym =~ /^[_A-Za-z]\w*$/
      class_eval("        @@#{sym} = nil unless defined? @@#{sym}

        def self.#{sym}
          @@#{sym}
        end
", __FILE__, __LINE__ + 1)

      unless options[:instance_reader] == false || options[:instance_accessor] == false
        class_eval("          def #{sym}
            @@#{sym}
          end
", __FILE__, __LINE__ + 1)
      end
      class_variable_set("@@#{sym}", yield) if block_given?
    end
  end

Defines a class attribute and creates a class and instance writer methods to allow assignment to the attribute.

module HairColors
  mattr_writer :hair_colors
end

class Person
  include HairColors
end

HairColors.hair_colors = [:brown, :black]
Person.class_variable_get("@@hair_colors") # => [:brown, :black]
Person.new.hair_colors = [:blonde, :red]
HairColors.class_variable_get("@@hair_colors") # => [:blonde, :red]

If you want to opt out the instance writer method, pass instance_writer: false or instance_accessor: false.

module HairColors
  mattr_writer :hair_colors, instance_writer: false
end

class Person
  include HairColors
end

Person.new.hair_colors = [:blonde, :red] # => NoMethodError

Also, you can pass a block to set up the attribute with a default value.

class HairColors
  mattr_writer :hair_colors do
    [:brown, :black, :blonde, :red]
  end
end

class Person
  include HairColors
end

Person.class_variable_get("@@hair_colors") # => [:brown, :black, :blonde, :red]

# File activesupport/lib/active_support/core_ext/module/attribute_accessors.rb, line 119
  def mattr_writer(*syms)
    options = syms.extract_options!
    syms.each do |sym|
      raise NameError.new("invalid attribute name: #{sym}") unless sym =~ /^[_A-Za-z]\w*$/
      class_eval("        @@#{sym} = nil unless defined? @@#{sym}

        def self.#{sym}=(obj)
          @@#{sym} = obj
        end
", __FILE__, __LINE__ + 1)

      unless options[:instance_writer] == false || options[:instance_accessor] == false
        class_eval("          def #{sym}=(obj)
            @@#{sym} = obj
          end
", __FILE__, __LINE__ + 1)
      end
      send("#{sym}=", yield) if block_given?
    end
  end

Returns the module which contains this one according to its name.

module M
  module N
  end
end
X = M::N

M::N.parent # => M
X.parent    # => M

The parent of top-level and anonymous modules is Object.

M.parent          # => Object
Module.new.parent # => Object

# File activesupport/lib/active_support/core_ext/module/introspection.rb, line 30
def parent
  parent_name ? ActiveSupport::Inflector.constantize(parent_name) : Object
end

Returns the name of the module containing this one.

M::N.parent_name # => "M"

# File activesupport/lib/active_support/core_ext/module/introspection.rb, line 7
def parent_name
  if defined? @parent_name
    @parent_name
  else
    @parent_name = name =~ /::[^:]+\Z/ ? $`.freeze : nil
  end
end

Returns all the parents of this module according to its name, ordered from nested outwards. The receiver is not contained within the result.

module M
  module N
  end
end
X = M::N

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

# File activesupport/lib/active_support/core_ext/module/introspection.rb, line 46
def parents
  parents = []
  if parent_name
    parts = parent_name.split('::')
    until parts.empty?
      parents << ActiveSupport::Inflector.constantize(parts * '::')
      parts.pop
    end
  end
  parents << Object unless parents.include? Object
  parents
end