Extends the module object with class/module and instance accessors for class/module attributes, just like the native attr* accessors for instance attributes.

Extends the module object with class/module and instance accessors for class/module attributes, just like the native attr* accessors for instance attributes, but does so on a per-thread basis.

So the values are scoped within the Thread.current space under the class name of the module.

Namespace

Module

Class

Methods

Included Modules

Constants

DELEGATION_RESERVED_KEYWORDS = %w(_ arg args block)
DELEGATION_RESERVED_METHOD_NAMES = Set.new( RUBY_RESERVED_KEYWORDS + DELEGATION_RESERVED_KEYWORDS ).freeze
RUBY_RESERVED_KEYWORDS = %w(alias and BEGIN begin break case class def defined? do else elsif END end ensure false for if in module next nil not or redo rescue retry return self super then true undef unless until when while yield)

Attributes

[RW] attr_internal_naming_format

Instance Public methods

alias_attribute(new_name, old_name)

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"
📝 Source code
# File activesupport/lib/active_support/core_ext/module/aliasing.rb, line 21
  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 <<-STR, __FILE__, __LINE__ + 1
      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
    STR
  end
🔎 See on GitHub

anonymous?()

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
📝 Source code
# File activesupport/lib/active_support/core_ext/module/anonymous.rb, line 27
  def anonymous?
    name.nil?
  end
🔎 See on GitHub

attr_internal(*attrs)

attr_internal_accessor(*attrs)

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

Also aliased as: attr_internal
📝 Source code
# File activesupport/lib/active_support/core_ext/module/attr_internal.rb, line 16
  def attr_internal_accessor(*attrs)
    attr_internal_reader(*attrs)
    attr_internal_writer(*attrs)
  end
🔎 See on GitHub

attr_internal_reader(*attrs)

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

📝 Source code
# File activesupport/lib/active_support/core_ext/module/attr_internal.rb, line 5
  def attr_internal_reader(*attrs)
    attrs.each { |attr_name| attr_internal_define(attr_name, :reader) }
  end
🔎 See on GitHub

attr_internal_writer(*attrs)

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

📝 Source code
# File activesupport/lib/active_support/core_ext/module/attr_internal.rb, line 10
  def attr_internal_writer(*attrs)
    attrs.each { |attr_name| attr_internal_define(attr_name, :writer) }
  end
🔎 See on GitHub

cattr_accessor(*syms, instance_reader: true, instance_writer: true, instance_accessor: true, default: nil, &blk)

Alias for: mattr_accessor

cattr_reader(*syms, instance_reader: true, instance_accessor: true, default: nil)

Alias for: mattr_reader

cattr_writer(*syms, instance_writer: true, instance_accessor: true, default: nil)

Alias for: mattr_writer

delegate(*methods, to: nil, prefix: nil, allow_nil: nil)

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. If you wish to instead return nil, use the :allow_nil option.

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.

📝 Source code
# File activesupport/lib/active_support/core_ext/module/delegation.rb, line 154
  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.map 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 = %(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
🔎 See on GitHub

delegate_missing_to(target)

When building decorators, a common pattern may emerge:

class Partition
  def initialize(event)
    @event = event
  end

  def person
    @event.detail.person || @event.creator
  end

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

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

With Module#delegate_missing_to, the above is condensed to:

class Partition
  delegate_missing_to :@event

  def initialize(event)
    @event = event
  end

  def person
    @event.detail.person || @event.creator
  end
end

The target can be anything callable within the object, e.g. instance variables, methods, constants, etc.

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

📝 Source code
# File activesupport/lib/active_support/core_ext/module/delegation.rb, line 258
  def delegate_missing_to(target)
    target = target.to_s
    target = "self.#{target}" if DELEGATION_RESERVED_METHOD_NAMES.include?(target)

    module_eval <<-RUBY, __FILE__, __LINE__ + 1
      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
          begin
            super
          rescue NoMethodError
            if #{target}.nil?
              raise DelegationError, "\#{method} delegated to #{target}, but #{target} is nil"
            else
              raise
            end
          end
        end
      end
    RUBY
  end
🔎 See on GitHub

deprecate(*method_names)

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
📝 Source code
# File activesupport/lib/active_support/core_ext/module/deprecation.rb, line 22
  def deprecate(*method_names)
    ActiveSupport::Deprecation.deprecate_methods(self, *method_names)
  end
🔎 See on GitHub

mattr_accessor(*syms, instance_reader: true, instance_writer: true, instance_accessor: true, default: nil, &blk)

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

You can set a default value for the attribute.

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

class Person
  include HairColors
end

Person.class_variable_get("@@hair_colors") # => [:brown, :black, :blonde, :red]
Also aliased as: cattr_accessor
📝 Source code
# File activesupport/lib/active_support/core_ext/module/attribute_accessors.rb, line 210
  def mattr_accessor(*syms, instance_reader: true, instance_writer: true, instance_accessor: true, default: nil, &blk)
    mattr_reader(*syms, instance_reader: instance_reader, instance_accessor: instance_accessor, default: default, &blk)
    mattr_writer(*syms, instance_writer: instance_writer, instance_accessor: instance_accessor, default: default)
  end
🔎 See on GitHub

mattr_reader(*syms, instance_reader: true, instance_accessor: true, default: nil)

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

You can set a default value for the attribute.

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

class Person
  include HairColors
end

Person.new.hair_colors # => [:brown, :black, :blonde, :red]
Also aliased as: cattr_reader
📝 Source code
# File activesupport/lib/active_support/core_ext/module/attribute_accessors.rb, line 54
  def mattr_reader(*syms, instance_reader: true, instance_accessor: true, default: nil)
    syms.each do |sym|
      raise NameError.new("invalid attribute name: #{sym}") unless /\A[_A-Za-z]\w*\z/.match?(sym)
      class_eval(<<-EOS, __FILE__, __LINE__ + 1)
        @@#{sym} = nil unless defined? @@#{sym}

        def self.#{sym}
          @@#{sym}
        end
      EOS

      if instance_reader && instance_accessor
        class_eval(<<-EOS, __FILE__, __LINE__ + 1)
          def #{sym}
            @@#{sym}
          end
        EOS
      end

      sym_default_value = (block_given? && default.nil?) ? yield : default
      class_variable_set("@@#{sym}", sym_default_value) unless sym_default_value.nil?
    end
  end
🔎 See on GitHub

mattr_writer(*syms, instance_writer: true, instance_accessor: true, default: nil)

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

You can set a default value for the attribute.

module HairColors
  mattr_writer :hair_colors, default: [:brown, :black, :blonde, :red]
end

class Person
  include HairColors
end

Person.class_variable_get("@@hair_colors") # => [:brown, :black, :blonde, :red]
Also aliased as: cattr_writer
📝 Source code
# File activesupport/lib/active_support/core_ext/module/attribute_accessors.rb, line 121
  def mattr_writer(*syms, instance_writer: true, instance_accessor: true, default: nil)
    syms.each do |sym|
      raise NameError.new("invalid attribute name: #{sym}") unless /\A[_A-Za-z]\w*\z/.match?(sym)
      class_eval(<<-EOS, __FILE__, __LINE__ + 1)
        @@#{sym} = nil unless defined? @@#{sym}

        def self.#{sym}=(obj)
          @@#{sym} = obj
        end
      EOS

      if instance_writer && instance_accessor
        class_eval(<<-EOS, __FILE__, __LINE__ + 1)
          def #{sym}=(obj)
            @@#{sym} = obj
          end
        EOS
      end

      sym_default_value = (block_given? && default.nil?) ? yield : default
      send("#{sym}=", sym_default_value) unless sym_default_value.nil?
    end
  end
🔎 See on GitHub

parent()

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
📝 Source code
# File activesupport/lib/active_support/core_ext/module/introspection.rb, line 34
  def parent
    parent_name ? ActiveSupport::Inflector.constantize(parent_name) : Object
  end
🔎 See on GitHub

parent_name()

Returns the name of the module containing this one.

M::N.parent_name # => "M"
📝 Source code
# File activesupport/lib/active_support/core_ext/module/introspection.rb, line 9
  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
🔎 See on GitHub

parents()

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]
📝 Source code
# File activesupport/lib/active_support/core_ext/module/introspection.rb, line 50
  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
🔎 See on GitHub

redefine_method(method, &block)

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

📝 Source code
# File activesupport/lib/active_support/core_ext/module/redefine_method.rb, line 26
  def redefine_method(method, &block)
    visibility = method_visibility(method)
    silence_redefinition_of_method(method)
    define_method(method, &block)
    send(visibility, method)
  end
🔎 See on GitHub

redefine_singleton_method(method, &block)

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

📝 Source code
# File activesupport/lib/active_support/core_ext/module/redefine_method.rb, line 35
  def redefine_singleton_method(method, &block)
    singleton_class.redefine_method(method, &block)
  end
🔎 See on GitHub

remove_possible_method(method)

Removes the named method, if it exists.

📝 Source code
# File activesupport/lib/active_support/core_ext/module/remove_method.rb, line 7
  def remove_possible_method(method)
    if method_defined?(method) || private_method_defined?(method)
      undef_method(method)
    end
  end
🔎 See on GitHub

remove_possible_singleton_method(method)

Removes the named singleton method, if it exists.

📝 Source code
# File activesupport/lib/active_support/core_ext/module/remove_method.rb, line 14
  def remove_possible_singleton_method(method)
    singleton_class.remove_possible_method(method)
  end
🔎 See on GitHub

silence_redefinition_of_method(method)

Marks the named method as intended to be redefined, if it exists. Suppresses the Ruby method redefinition warning. Prefer redefine_method where possible.

📝 Source code
# File activesupport/lib/active_support/core_ext/module/redefine_method.rb, line 8
    def silence_redefinition_of_method(method)
      if method_defined?(method) || private_method_defined?(method)
        # This suppresses the "method redefined" warning; the self-alias
        # looks odd, but means we don't need to generate a unique name
        alias_method method, method
      end
    end
🔎 See on GitHub

thread_cattr_accessor(*syms)

thread_mattr_accessor(*syms)

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
Also aliased as: thread_cattr_accessor
📝 Source code
# File activesupport/lib/active_support/core_ext/module/attribute_accessors_per_thread.rb, line 145
  def thread_mattr_accessor(*syms)
    thread_mattr_reader(*syms)
    thread_mattr_writer(*syms)
  end
🔎 See on GitHub