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