mongoid_belongs_to_extension.rb

#
# To use this, drop it into your lib directory and add a require to an initializer
# in config/initializers.
#
# This little extension to Mongoid allows belongs_to_related associations to
# have an "in" option, which will traverse the object graph to find the
# collection that will contain the related model object.
#
# The goal is to allow relations to be created that reference objects embedded
# in documents, instead of only objects that are contained within collections.
#
# For instance, if you have an object model defined as such (totally contrived):
#
#    class Blog
#      ...
#      embeds_many :authors
#      embeds_many :posts
#    end
#  
#    class Author
#      ...
#      embedded_in :blog, :inverse_of => :authors
#      embeds_many :pictures
#    end
#
#    class Picture
#      ...
#      embedded_in :author, :inverse_of => :pictures
#    end
#
#    class Post
#      ...
#      embedded_in :blog, :inverse_of => :posts
#      belongs_to_related :author, :in => :blog
#    end
#
# post.author can now be used to relate to any author within Blog. The accessor
# uses the blog method on the Post instance and looks for an authors method on
# whatever the blog method returned.
#
# For relations on objects that are embedded several layers deep, an array can
# be passed to :in, and it will traverse the objects, calling the passed methods
# in order to arrive at the object with the accessor that will return the embedded
# collection that will contain the object we're looking for. Phew.
#

module Mongoid
  module Associations
    class Options
      def in
        @attributes[:in]
      end
    end
  end
end

module Mongoid
  module Associations
    class BelongsToRelated < Proxy
      def initialize(document, foreign_key, options, target = nil)
        @options = options
        
        if target == nil and options.in
          if options.in.is_a?(Array)
            # If in is an array, traverse the object graph to find the
            # right container.
            container = options.in.inject(document) do |obj, sym|
              obj.send(sym)
            end
          else
            # If not, just access it directly
            container = document.send(options.in)
          end
          
          okdcpts = options.klass.name.downcase.pluralize.to_sym
          
          # Look for a has_many style plural method
          if container.respond_to?(okdcpts)
            @target = container.send(okdcpts).find(foreign_key)
          else
            raise "belongs_to_related :in target does not contain the method :#{okdcpts.to_s}"
          end
        else
          @target = target || options.klass.find(foreign_key)
        end
        
        extends(options)
      end
    end
  end
end