# File activesupport/lib/active_support/core_ext/module/aliasing.rb, line 19
  def alias_attribute(new_name, old_name)
    # The following reader methods use an explicit `self` receiver in order to
    # support aliases that start with an uppercase letter. Otherwise, they would
    # be resolved as constants instead.
    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

Allows you to make aliases for attributes, which includes getter, setter, and a predicate.

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/anonymous.rb, line 25
def anonymous?
  name.nil?
end

A module may or may not have a name.

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

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

anonymous? method returns true if module does not have a name, false otherwise:

Module.new.anonymous? # => true

module M; end
M.anonymous?          # => false

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.anonymous?   # => true
M = m          # m gets a name here as a side-effect
m.name         # => "M"
m.anonymous?   # => false

# 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 and writer 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 reader 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

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

# File activesupport/lib/active_support/core_ext/module/delegation.rb, line 155
def delegate(*methods, to: nil, prefix: nil, allow_nil: nil)
  unless 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

  if prefix == true && /^[^a-z_]/.match?(to)
    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

  location = caller_locations(1, 1).first
  file, line = location.path, location.lineno

  to = to.to_s
  to = "self.#{to}" if DELEGATION_RESERVED_METHOD_NAMES.include?(to)

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

    # The following generated method calls 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})",
        "_ = #{to}",
        "if !_.nil? || nil.respond_to?(:#{method})",
        "  _.#{method}(#{definition})",
        "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})",
        " _ = #{to}",
        "  _.#{method}(#{definition})",
        "rescue NoMethodError => e",
        "  if _.nil? && e.name == :#{method}",
        "    #{exception}",
        "  else",
        "    raise",
        "  end",
        "end"
      ].join ";"
    end

    module_eval(method_def, file, line)
  end
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 Module::DelegationError from being 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 Module::DelegationError 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
# => Module::DelegationError: User#age delegated to profile.age, but profile is nil

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 264
  def delegate_missing_to(target)
    target = target.to_s
    target = "self.#{target}" if DELEGATION_RESERVED_METHOD_NAMES.include?(target)

    module_eval "      def respond_to_missing?(name, include_private = false)
        # It may look like an oversight, but we deliberately do not pass
        # +include_private+, because they do not get delegated.

        #{target}.respond_to?(name) || super
      end

      def method_missing(method, *args, &block)
        if #{target}.respond_to?(method)
          #{target}.public_send(method, *args, &block)
        else
          super
        end
      end
", __FILE__, __LINE__ + 1
  end

When building decorators, a common pattern may emerge:

class Partition
  def initialize(first_event)
    @events = [ first_event ]
  end

  def people
    if @events.first.detail.people.any?
      @events.collect { |e| Array(e.detail.people) }.flatten.uniq
    else
      @events.collect(&:creator).uniq
    end
  end

  private
    def respond_to_missing?(name, include_private = false)
      @events.respond_to?(name, include_private)
    end

    def method_missing(method, *args, &block)
      @events.send(method, *args, &block)
    end
end

With `Module#delegate_missing_to`, the above is condensed to:

class Partition
  delegate_missing_to :@events

  def initialize(first_event)
    @events = [ first_event ]
  end

  def people
    if @events.first.detail.people.any?
      @events.collect { |e| Array(e.detail.people) }.flatten.uniq
    else
      @events.collect(&:creator).uniq
    end
  end
end

The target can be anything callable within the object. E.g. instance variables, methods, constants and the likes.

# File activesupport/lib/active_support/core_ext/module/deprecation.rb, line 20
def deprecate(*method_names)
  ActiveSupport::Deprecation.deprecate_methods(self, *method_names)
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/attribute_accessors.rb, line 213
def mattr_accessor(*syms, &blk)
  mattr_reader(*syms, &blk)
  mattr_writer(*syms)
end

Defines both class and instance accessors for class attributes. All class and instance methods created will be public, even if this method is called with a private or protected access modifier.

module HairColors
  mattr_accessor :hair_colors
end

class Person
  include HairColors
end

HairColors.hair_colors = [:brown, :black, :blonde, :red]
HairColors.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.new.hair_colors << :blue
Person.new.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 55
  def mattr_reader(*syms)
    options = syms.extract_options!
    syms.each do |sym|
      raise NameError.new("invalid attribute name: #{sym}") unless /\A[_A-Za-z]\w*\z/.match?(sym)
      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 reader methods. The underlying class variable is set to nil, if it is not previously defined. All class and instance methods created will be public, even if this method is called with a private or protected access modifier.

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: 1_Badname

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

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

class Person
  include HairColors
end

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

# File activesupport/lib/active_support/core_ext/module/attribute_accessors.rb, line 123
  def mattr_writer(*syms)
    options = syms.extract_options!
    syms.each do |sym|
      raise NameError.new("invalid attribute name: #{sym}") unless /\A[_A-Za-z]\w*\z/.match?(sym)
      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

Defines a class attribute and creates a class and instance writer methods to allow assignment to the attribute. All class and instance methods created will be public, even if this method is called with a private or protected access modifier.

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.

module 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/introspection.rb, line 32
def parent
  parent_name ? ActiveSupport::Inflector.constantize(parent_name) : Object
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 7
def parent_name
  if defined?(@parent_name)
    @parent_name
  else
    parent_name = name =~ /::[^:]+\Z/ ? $`.freeze : nil
    @parent_name = parent_name unless frozen?
    parent_name
  end
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 48
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

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/remove_method.rb, line 18
def redefine_method(method, &block)
  visibility = method_visibility(method)
  remove_possible_method(method)
  define_method(method, &block)
  send(visibility, method)
end

Replaces the existing method definition, if there is one, with the passed block as its body.

# File activesupport/lib/active_support/core_ext/module/remove_method.rb, line 3
def remove_possible_method(method)
  if method_defined?(method) || private_method_defined?(method)
    undef_method(method)
  end
end

Removes the named method, if it exists.

# File activesupport/lib/active_support/core_ext/module/remove_method.rb, line 10
def remove_possible_singleton_method(method)
  singleton_class.instance_eval do
    remove_possible_method(method)
  end
end

Removes the named singleton method, if it exists.

# File activesupport/lib/active_support/core_ext/module/attribute_accessors_per_thread.rb, line 143
def thread_mattr_accessor(*syms)
  thread_mattr_reader(*syms)
  thread_mattr_writer(*syms)
end

Defines both class and instance accessors for class attributes.

class Account
  thread_mattr_accessor :user
end

Account.user = "DHH"
Account.user     # => "DHH"
Account.new.user # => "DHH"

If a subclass changes the value, the parent class' value is not changed. Similarly, if the parent class changes the value, the value of subclasses is not changed.

class Customer < Account
end

Customer.user = "Rafael"
Customer.user # => "Rafael"
Account.user  # => "DHH"

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

class Current
  thread_mattr_accessor :user, instance_writer: false, instance_reader: false
end

Current.new.user = "DHH"  # => NoMethodError
Current.new.user          # => NoMethodError

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

class Current
  mattr_accessor :user, instance_accessor: false
end

Current.new.user = "DHH"  # => NoMethodError
Current.new.user          # => NoMethodError