Class: Module

Alias for #attr_internal_accessor.

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

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

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

Alias for #mattr_accessor.

Alias for #mattr_reader.

Alias for #mattr_writer.

Returns a copy of module or class if it's anonymous. If it's named, returns self.

Object.deep_dup == Object # => true
klass = Class.new
klass.deep_dup == klass # => false

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

Options

• :to - Specifies the target object name as a symbol or string
• :prefix - Prefixes the new method with the target name or a custom prefix
• :allow_nil - If set to true, prevents a ::ActiveSupport::DelegationError from being raised
• :private - If set to true, changes method visibility to private

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'

The delegated methods are public by default.

Pass private: true to change that.

class User < {ActiveRecord::Base}
  has_one :profile
  delegate :first_name, to: :profile
  delegate :date_of_birth, to: :profile, private: true

  def age
    Date.today.year - date_of_birth.year
  end
end

{User.new}.first_name # => "Tomas"
{User.new}.date_of_birth # => NoMethodError: private method `date_of_birth' called for {#<}User:0x00000008221340>
{User.new}.age # => 2

If the target is nil and does not respond to the delegated method a

::ActiveSupport::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
#### => {ActiveSupport::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.

When building decorators, a common pattern may emerge:

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

  def person
    detail.person || 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 delegate_missing_to, the above is condensed to:

class Partition
  delegate_missing_to :@event

  def initialize(event)
    @event = event
  end

  def person
    detail.person || 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 ::ActiveSupport::DelegationError. If you wish to instead return nil, use the :allow_nil option.

The marshal_dump and _dump methods are exempt from delegation due to possible interference when calling Marshal.dump(object), should the delegation target method of object add or remove instance variables.

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

A deprecator is typically an instance of ::ActiveSupport::Deprecation, but you can also pass any object that responds to deprecation_warning(deprecated_method_name, message, caller_backtrace) 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

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 Citizen < Person
end

{Citizen.new}.hair_colors << :blue
{Person.new}.hair_colors # => [:brown, :black, :blonde, :red, :blue]

To omit the instance writer method, pass instance_writer: false.

To omit 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 omit 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]
  mattr_accessor(:hair_styles) { [:long, :short] }
end

class Person
  include HairColors
end

{Person.class_variable_get}("@@hair_colors") # => [:brown, :black, :blonde, :red]
{Person.class_variable_get}("@@hair_styles") # => [:long, :short]

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

To omit 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]
  mattr_reader(:hair_styles) { [:long, :short] }
end

class Person
  include HairColors
end

{Person.new}.hair_colors # => [:brown, :black, :blonde, :red]
{Person.new}.hair_styles # => [:long, :short]

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]

To omit 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]
  mattr_writer(:hair_styles) { [:long, :short] }
end

class Person
  include HairColors
end

{Person.class_variable_get}("@@hair_colors") # => [:brown, :black, :blonde, :red]
{Person.class_variable_get}("@@hair_styles") # => [:long, :short]

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

module M
  module N
  end
end
X = {M::N}

{M::N.module_parent} # => M
{X.module_parent}    # => M

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

{M.module_parent}          # => Object
{Module.new}.module_parent # => Object

Returns the name of the module containing this one.

{M::N.module_parent_name} # => "M"

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.module_parents}    # => [Object]
{M::N.module_parents} # => [M, Object]
{X.module_parents}    # => [M, Object]

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

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

Removes the named method, if it exists.

Removes the named singleton method, if it exists.

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

Alias for #thread_mattr_accessor.

Alias for #thread_mattr_reader.

Alias for #thread_mattr_writer.

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"

Unlike #mattr_accessor, values are not shared with subclasses or parent classes.

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

class Customer < Account
end

{Account.user}   # => "DHH"
{Customer.user}  # => nil
{Customer.user}  = "Rafael"
{Customer.user}  # => "Rafael"
{Account.user}   # => "DHH"

To omit the instance writer method, pass instance_writer: false.

To omit 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 omit both instance methods.

class Current
  thread_mattr_accessor :user, instance_accessor: false
end

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

A default value may be specified using the :default option. Because

multiple threads can access the default value, non-frozen default values will be duped and frozen.

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

module Current
  thread_mattr_reader :user
end

{Current.user} = "DHH"
{Current.user} # => "DHH"
{Thread.new} { {Current.user} }.value # => nil

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

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

To omit the instance reader method, pass

instance_reader: false or instance_accessor: false.

class Current
  thread_mattr_reader :user, instance_reader: false
end

{Current.new}.user # => NoMethodError

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

module Current
  thread_mattr_writer :user
end

{Current.user} = "DHH"
Thread.current[:attr_Current_user] # => "DHH"

To omit the instance writer method, pass

instance_writer: false or instance_accessor: false.

class Current
  thread_mattr_writer :user, instance_writer: false
end

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