Question

In my app I am having to interface with some 3rd party software I hope one day to replace. So, rather than keeping all the code that maps between my models’ data and the form it needs to be in for the 3rd party software in the models themselves, I’ve created a mapper module for each model, isolating the code somewhere that’s easy to delete when the time comes.

So I have something like the following:

app/
  models/
    people.rb
  mappers/
    people_mapper.rb

Ideally, I’d like to automatically include the modules in the model class with the matching name, the same way that helpers are automatically included in views of the same name. How/where are the helpers automatically included, and is this also the best place for me to add my own code?

Answer 1

you can try something like this :

module Mapper::Core

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

  module ClassMethods

    # model class method to include matching module
    # this will throw an error if matching class constant name does not exist
    def has_mapping
      @mapper_class = Kernel.const_get( "Mapper::#{self}Mapper" )
      include @mapper_class
    end

    # an accessor to the matching class mapper may come in handy
    def mapper_class
      @mapper_class
    end

  end
end     

then then require and include the module in ActiveRecord::Base in an initializer (make sure that your Mapper module requires all the files in your ‘mappers’ folder, or use config.autoload_paths).

If you don’t want to use the has_mapping class method at all, you can try to override ActiveRecord::Base‘s self.inherited callback, but it might become dangerous:

  def self.included( base )
    base.extend( ClassMethods )
    base.instance_eval <<-EOF
      alias :old_inherited :inherited
      def self.inherited( subclass )
        subclass.has_mapping
        old_inherited( subclass )
      end
    EOF
  end

I did not try any of this, so proceed with caution.

EDIT :

i was tired when i wrote this. there is a much simpler way to autoinclude the matching module :

module Mapper::Core
  def self.included( base )
    begin
      mapper_class = Kernel.const_get( "Mapper::#{base.name}Mapper" )
      base.instance_eval( "include #{mapper_class}" ) 
    rescue 
      Logger.info "No matching Mapper Class found for #{base.name}"
    end
  end
end

initialize this with :

ActiveRecord::base.instance_eval( 'include Mapper::Core' )

all inheriting class will now include Mapper::Core, which will trigger inclusion of matching class.