Question

I have a class but I want some customized behavior if a field is of a particular value. Otherwise I want the default behavior.

class Foo
  include Mongoid::Document

  belongs_to :parent, :foreign_key => "parent_id", :class_name => "Pad"
  has_many :children, :foreign_key => "parent_id", :class_name => "Pad"
  field :bar, :type => String

  def children
    if self.bar == "some value"
      # Do something special
    else
      return self.children   # <- What goes here that isn't an infinite loop?
    end
  end
end

What should the else branch be?

I do not want to reimplement children so I’m looking for more than just Foo.where(:parent_id => self.id)

Answer 1

Active Support provides a method to keep the original method around for cases like this — it’s called “alias_method_chain”… It’s a bit of black magic, but you define the {method}_with_{condition} and call the original with {method}_without_{condition}

In your case: children_with_bar and you get to the original by calling children_without_bar

require 'active_support'
require 'mongoid'

class Foo
  include Mongoid::Document
  has_many :children, :foreign_key => "parent_id", :class_name => "Pad"
  field :bar

  def children_with_bar
    if self.bar == "some value"
      return "yooooo"
    else
      children_without_bar
    end
  end

  alias_method_chain :children, :bar
end


Foo.new(:bar=>"some value").children
 => "yooooo" 
Foo.new(:bar=>"different").children
 ## Will run original children method