active record中delegate方法文档

delegate(*methods) Link
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 NoMethodError to be 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 #
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 NoMethodError 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 # raises NoMethodError: undefined method `age’
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.