instance method
delegate
Ruby on Rails 4.0.13
Signature
delegate(*methods)Provides a delegate class method to easily expose contained objects’ public methods as your own.
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 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'
Parameters
-
methodsrest
Source
# File activesupport/lib/active_support/core_ext/module/delegation.rb, line 132
def delegate(*methods)
options = methods.pop
unless options.is_a?(Hash) && to = options[: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
prefix, allow_nil = options.values_at(:prefix, :allow_nil)
if prefix == true && to =~ /^[^a-z_]/
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
file, line = caller.first.split(':', 2)
line = line.to_i
to = to.to_s
to = 'self.class' if to == 'class'
methods.each do |method|
# Attribute writer methods only accept one argument. Makes sure []=
# methods still accept two arguments.
definition = (method =~ /[^\]]=$/) ? 'arg' : '*args, &block'
# The following generated methods call 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 conceptualy, from the user point of view, the delegator should
# be doing one call.
if allow_nil
module_eval(<<-EOS, file, line - 3)
def #{method_prefix}#{method}(#{definition}) # def customer_name(*args, &block)
_ = #{to} # _ = client
if !_.nil? || nil.respond_to?(:#{method}) # if !_.nil? || nil.respond_to?(:name)
_.#{method}(#{definition}) # _.name(*args, &block)
end # end
end # end
EOS
else
exception = %(raise "#{self}##{method_prefix}#{method} delegated to #{to}.#{method}, but #{to} is nil: \#{self.inspect}")
module_eval(<<-EOS, file, line - 2)
def #{method_prefix}#{method}(#{definition}) # def customer_name(*args, &block)
_ = #{to} # _ = client
_.#{method}(#{definition}) # _.name(*args, &block)
rescue NoMethodError => e # rescue NoMethodError => e
location = "%s:%d:in `%s'" % [__FILE__, __LINE__ - 2, '#{method_prefix}#{method}'] # location = "%s:%d:in `%s'" % [__FILE__, __LINE__ - 2, 'customer_name']
if _.nil? && e.backtrace.first == location # if _.nil? && e.backtrace.first == location
#{exception} # # add helpful message to the exception
else # else
raise # raise
end # end
end # end
EOS
end
end
end
Defined in activesupport/lib/active_support/core_ext/module/delegation.rb line 132
· View on GitHub
· Improve this page
· Find usages on GitHub
Defined in Module