README.markdown

MongoMapper does not include HABTM relationships. This is my attempt to add them.

node.rb

class Node
  include MongoMapper::Document
  include Relationship

  key :is_published, Boolean, :default => false
  key :title, String, :required => true

  # provides Node#terms, Node#save_taggings
  habtm :nodes, :terms, :tagging

  after_save :save_taggings
end

relationship.rb

module Relationship
  module InstanceMethods
  end

  module ClassMethods
    def habtm(this_model, that_model, glue_model, options = {})
      define_method Private::plural(that_model) do
        @those_models ||= Private::klass(glue_model).send("#{Private::plural(that_model)}_for", self.id)
      end

      define_method "save_#{Private::plural(glue_model)}" do
        @those_models ||= []
        @those_models.each { |om| Private::klass(glue_model).first_or_create(Private::id_key(this_model) => self.id, Private::id_key(that_model) => om.id) }
      end
    end

    def habtm_glue(model_a, model_b, options = {})
      {
        model_a => model_b,
        model_b => model_a
      }.each do |this_model, that_model|
        key Private::id_key(this_model), BSON::ObjectID, :required => true

        define_method this_model do
          Private::klass(this_model).find(self.send(Private::id_key(this_model)))
        end

        define_method "#{this_model}=" do |new_model|
          self.send "#{Private::id_key(this_model)}=", new_model.id
          new_model
        end

        this_model_for = lambda { |that_model_id| all(Private::id_key(that_model) => that_model_id).map { |tagging| tagging.send(this_model) } }
        self.class.send(:define_method, "#{Private::plural(this_model)}_for", this_model_for)
      end
    end
  end

  module Private
    def self.single(model_name)
      model_name.to_s.singularize.to_sym
    end

    def self.plural(model_name)
      model_name.to_s.pluralize.to_sym
    end

    def self.id_key(model_name)
      "#{single(model_name)}_id".to_sym
    end

    def self.klass(model_name)
      single(model_name).to_s.capitalize.constantize
    end
  end

  def self.included(base)
    base.send :extend, ClassMethods
    base.send :include, InstanceMethods
  end
end

tagging.rb

class Tagging
  include MongoMapper::Document
  include Relationship

  # provides Tagging#node, Tagging#term, Tagging#node=, Tagging#term=, Tagging.terms_for(node_id), Tagging.node_for(term_id)
  habtm_glue :node, :term
end

term.rb

class Term
  include MongoMapper::Document
  include Relationship

  key :name, String, :required => true

  # provides Term#nodes, Term#save_nodes
  habtm :terms, :nodes, :tagging

  after_save :save_taggings
end