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#1127 ✓resolved

:allow_nil option for delegate

Reported by porras | September 27th, 2008 @ 08:43 PM | in 2.x

Adds :allow_nil option to delegate method:

  # If the object in which you delegate can be nil, you may want to use the
  # :allow_nil option. In that case, it returns nil instead of raising a
  # NoMethodError exception:
  #  class Foo
  #    attr_accessor :bar
  #    def initialize(bar = nil)
  #      @bar = bar
  #    end
  #    delegate :zoo, :to => :bar
  #  end
  #   # raises NoMethodError exception (you called nil.zoo)
  #  class Foo
  #    attr_accessor :bar
  #    def initialize(bar = nil)
  #      @bar = bar
  #    end
  #    delegate :zoo, :to => :bar, :allow_nil => true
  #  end
  #   # returns nil

Comments and changes to this ticket

  • Daniel Schierbeck

    Daniel Schierbeck September 29th, 2008 @ 02:23 PM

    +1: This seems like a good addition to delegate, and I like the simplicity of the implementation.

  • Emili Parreño
  • David Röthlisberger

    David Röthlisberger October 14th, 2008 @ 02:06 PM

    1. Useful, and the patch is very simple.

    If anything, I'd change the rdoc to say "If the object to which you delegate...".

  • David Röthlisberger

    David Röthlisberger October 14th, 2008 @ 02:07 PM

    (Um, that "1." in the comment above was supposed to be a "+1").

  • Josh Susser

    Josh Susser October 14th, 2008 @ 03:30 PM


    I dislike silently hiding errors like that. Seems like just asking for trouble.

  • David Röthlisberger

    David Röthlisberger October 14th, 2008 @ 04:46 PM

    How is it hiding the error any more than:

    def contact_name; contact &&; end

    It's intended as a shortcut for the case where not having a contact isn't an error. Remember you have to specify :allow_nil, this isn't going to be default behaviour.

  • porras

    porras December 8th, 2008 @ 12:09 AM

    I updated the patch, which no longer applied after the :prefix option was added.

    @Josh, as Dave explained, this option is intended only for the cases where this is not an error but a legitimate situation, just to avoid that verbose and ugly definition.

  • Daniel Schierbeck

    Daniel Schierbeck December 16th, 2008 @ 07:02 PM

    I'd like some more attention brought on this ticket (i.e. BUMP), as I've recently found use cases for it abound.

    I currently have the following simplified setup:

    # A resident may or may not have a current residentship -- he
    # could be a former resident.
    class Resident < ActiveRecord::Base
      has_many :residentships
      delegate :room_number, :room, :to => :residentship
      def residentship
    class Residentship < ActiveRecord::Base
      belongs_to :resident
      validates_presence_of :room_number, :moved_out_at
      named_scope :only_current, ...
      composed_of :room, :mapping => %w(room_number number)
    class Room # value class, as there's a finite set of rooms
      def residentship
      delegate :resident, :to => :residentships

    I'd like Room#resident, Resident#room and Resident#room_number (as well as several more in my real app) to work properly for former residents, i.e. just return nil -- but with the current implementation of delegate, they raise an exception instead. Adding an :allow_nil option to delegate would greatly reduce the number of extra methods i need to have.

  • Repository

    Repository December 22nd, 2008 @ 12:14 AM

    • State changed from “new” to “resolved”

    (from [e8de7a67a5ef063164da022845a7cae1753da80e]) Add :allow_nil option to delegate [#1127 state:resolved]

    Signed-off-by: Pratik Naik

  • Ryan Bigg

    Ryan Bigg October 9th, 2010 @ 09:49 PM

    • Tag cleared.

    Automatic cleanup of spam.

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  • yoyogo

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