[Talking-Ruby] ActiveSupport Delegate module implement the delegation pattern

Posted on 2021-07-31 Edited on 2024-09-12 In Programming Language , Ruby , Talking-Rails Views: Word count in article: 5k Reading time ≈ 5 mins.

If your project includes ActiveSupport, and every Rails project does, you have a more clean and easy way to implement the delegation pattern: the Module#delegate extension. It provides a delegate module you can use in your class or in your modules to delegate a specific method to an associate object.

#delegate provides a delegate class method to easily expose contained objects’ public methods as your own.

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

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 Module::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 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'