2 files changed, 120 insertions, 0 deletions
diff --git a/activerecord/lib/active_record/associations/builder/association.rb b/activerecord/lib/active_record/associations/builder/association.rb
index 6cf5523ad6..783a86d4a3 100644
--- a/activerecord/lib/active_record/associations/builder/association.rb
+++ b/activerecord/lib/active_record/associations/builder/association.rb
@@ -37,6 +37,17 @@ module ActiveRecord::Associations::Builder
       reflection
     end
 
+    def self.create_builder(model, name, scope, options, &block)
+      raise ArgumentError, "association names must be a Symbol" unless name.kind_of?(Symbol)
+
+      if scope.is_a?(Hash)
+        options = scope
+        scope   = nil
+      end
+
+      new(name, scope, options, &block)
+    end
+
     def initialize(name, scope, options)
       @name    = name
       @scope   = scope
diff --git a/activerecord/lib/active_record/associations/builder/has_and_belongs_to_many.rb b/activerecord/lib/active_record/associations/builder/has_and_belongs_to_many.rb
index 50c9dbe73f..4b47824148 100644
--- a/activerecord/lib/active_record/associations/builder/has_and_belongs_to_many.rb
+++ b/activerecord/lib/active_record/associations/builder/has_and_belongs_to_many.rb
@@ -1,4 +1,113 @@
 module ActiveRecord::Associations::Builder
+  class HABTM
+    class JoinTableResolver
+      KnownTable = Struct.new :join_table
+
+      class KnownClass
+        def initialize(rhs_class, lhs_class_name)
+          @rhs_class      = rhs_class
+          @lhs_class_name = lhs_class_name
+          @join_table     = nil
+        end
+
+        def join_table
+          @join_table ||= [@rhs_class.table_name, klass.table_name].sort.join("\0").gsub(/^(.*_)(.+)\0\1(.+)/, '\1\2_\3').gsub("\0", "_")
+        end
+
+        private
+        def klass; @lhs_class_name.constantize; end
+      end
+
+      def self.build(rhs_class, name, options)
+        if options[:join_table]
+          KnownTable.new options[:join_table]
+        else
+          class_name = options.fetch(:class_name) {
+            name.to_s.camelize.singularize
+          }
+          KnownClass.new rhs_class, class_name
+        end
+      end
+    end
+
+    attr_reader :lhs_model, :association_name, :options
+
+    def initialize(association_name, lhs_model, options)
+      @association_name = association_name
+      @lhs_model = lhs_model
+      @options = options
+    end
+
+    def through_model
+      habtm = JoinTableResolver.build lhs_model, association_name, options
+
+      join_model = Class.new(ActiveRecord::Base) {
+        class << self;
+          attr_accessor :class_resolver
+          attr_accessor :name
+          attr_accessor :table_name_resolver
+          attr_accessor :left_association_name
+          attr_accessor :right_association_name
+        end
+
+        def self.table_name
+          table_name_resolver.join_table
+        end
+
+        def self.compute_type(class_name)
+          class_resolver.compute_type class_name
+        end
+
+        def self.add_left_association(name, options)
+          self.left_association_name = name
+          belongs_to name, options
+        end
+
+        def self.add_right_association(name, options)
+          rhs_name = name.to_s.singularize.to_sym
+          self.right_association_name = rhs_name
+          belongs_to rhs_name, options
+        end
+
+      }
+
+      join_model.name                = "HABTM_#{association_name.to_s.camelize}"
+      join_model.table_name_resolver = habtm
+      join_model.class_resolver      = lhs_model
+
+      join_model.add_left_association :left_side, class: lhs_model
+      join_model.add_right_association association_name, belongs_to_options(options)
+      join_model
+    end
+
+    def middle_options(join_model)
+      middle_options = {}
+      middle_options[:class] = join_model
+      middle_options[:source] = join_model.left_association_name
+      if options.key? :foreign_key
+        middle_options[:foreign_key] = options[:foreign_key]
+      end
+      middle_options
+    end
+
+    private
+
+    def belongs_to_options(options)
+      rhs_options = {}
+
+      if options.key? :class_name
+        rhs_options[:foreign_key] = options[:class_name].foreign_key
+        rhs_options[:class_name] = options[:class_name]
+      end
+
+      if options.key? :association_foreign_key
+        rhs_options[:foreign_key] = options[:association_foreign_key]
+      end
+
+      rhs_options
+    end
+  end
+
   class HasAndBelongsToMany < CollectionAssociation #:nodoc:
     def macro
       :has_and_belongs_to_many