Associates models to a parent by other methods, similar to delegates

associate_by.rb

# AssociateBy
#
# Associates models to a parent by other methods, similar to delegates but
# changing the association itself instead of the associated object when
# writing to the delegated attribute.
#
# Usage:
# ActiveRecord::Base.class_eval { include AssociateBy }
#
# Example:
#
# class Street < ActiveRecord::Base
#   has_many :locations
# end
#
# class Location < ActiveRecord::Base
#   belongs_to :street
#   associate :street, :by => :name
# end
#
# @street = Street.create :name => "First Street"
# puts @street.id #==> 1
# Street.create :name => "Second Ave"
#
# puts @location.street_name #==> nil
# @location.street_name = "First Street"
# puts @location.street.id #==> 1
#
# @location.street_name = "Second Ave"
# puts @location.street.id #==> 2
#
# @location.street = Street.first
# puts @location.street_name #==> "First Street"
#
# written by Kai Krakow, http://github.com/kakra
# Provided as-is

module AssociateBy

  def self.included(base)
    base.extend ClassMethods
  end

  module ClassMethods

    # TODO check sanity of parameters
    # TODO check "by_method" is an attribute of the association
    # TODO check "by_method" is a unique key of the association
    # TODO watch changes to the association to invalidate memoization
    def associate(association, options = {})
      by_method = options[:by].to_s
      associated_method = "#{association.to_s}_#{by_method}"
      src = <<-SRC
        def #{associated_method}
          @#{associated_method} ||= #{association.to_s}.#{by_method}
        rescue NoMethodError
          @#{associated_method} = nil
        end

        def #{associated_method}=(#{by_method})
          if self.#{association.to_s} = #{association.to_s.classify}.find_by_#{by_method}(#{by_method})
            @#{associated_method} = #{association}.#{by_method}
          else
            @#{associated_method} = nil
          end
        end
      SRC
      self.class_eval src, __FILE__, __LINE__
    end
  end
end