Pass the attribute and option hashes to build_association

The build_association method was added as an API for plugins
to hook into in 1398db0. This commit restores this API and the
ability to override class.new to return a subclass based on
a virtual attribute in the attributes hash.

activerecord/lib/active_record/associations/collection_association.rb

@@ -423,9 +423,13 @@ def insert_record(record, validate = true)
           raise NotImplementedError
         end
 
+        def create_scope
+          scoped.scope_for_create.stringify_keys
+        end
+
         def build_record(attributes, options)
-          record = reflection.build_association
-          record.assign_attributes(scoped.scope_for_create, :without_protection => true)
+          record = reflection.build_association(attributes, options)
+          record.assign_attributes(create_scope.except(*record.changed), :without_protection => true)
           record.assign_attributes(attributes, options)
           record
         end

activerecord/lib/active_record/associations/singular_association.rb

@@ -26,18 +26,18 @@ def create!(attributes = {}, options = {})
       end
 
       def build(attributes = {}, options = {})
-        record = reflection.build_association
-        record.assign_attributes(
-          scoped.scope_for_create.except(klass.primary_key),
-          :without_protection => true
-        )
-        record.assign_attributes(attributes, options)
+        record = reflection.build_association(attributes, options)
+        record.assign_attributes(create_scope.except(*record.changed), :without_protection => true)
         set_new_record(record)
         record
       end
 
       private
 
+        def create_scope
+          scoped.scope_for_create.stringify_keys.except(klass.primary_key)
+        end
+
         def find_target
           scoped.first.tap { |record| set_inverse_instance(record) }
         end

activerecord/test/cases/associations/has_many_associations_test.rb

@@ -1455,4 +1455,17 @@ def test_merging_with_custom_attribute_writer
 
     assert_equal "RED!", car.bulbs.to_a.first.color
   end
+
+  def test_new_is_called_with_attributes_and_options
+    car = Car.create(:name => 'honda')
+
+    bulb = car.bulbs.build
+    assert_equal Bulb, bulb.class
+
+    bulb = car.bulbs.build(:bulb_type => :custom)
+    assert_equal Bulb, bulb.class
+
+    bulb = car.bulbs.build({ :bulb_type => :custom }, :as => :admin)
+    assert_equal CustomBulb, bulb.class
+  end
 end

activerecord/test/cases/associations/has_one_associations_test.rb

@@ -410,4 +410,20 @@ def test_association_conditions_bypass_attribute_protection
     bulb = car.create_frickinawesome_bulb(:frickinawesome => false)
     assert_equal true, bulb.frickinawesome?
   end
+
+  def test_new_is_called_with_attributes_and_options
+    car = Car.create(:name => 'honda')
+
+    bulb = car.build_bulb
+    assert_equal Bulb, bulb.class
+
+    bulb = car.build_bulb
+    assert_equal Bulb, bulb.class
+
+    bulb = car.build_bulb(:bulb_type => :custom)
+    assert_equal Bulb, bulb.class
+
+    bulb = car.build_bulb({ :bulb_type => :custom }, :as => :admin)
+    assert_equal CustomBulb, bulb.class
+  end
 end

activerecord/test/models/bulb.rb

@@ -15,4 +15,17 @@ def color=(color)
     self[:color] = color.upcase + "!"
   end
 
+  def self.new(attributes = {}, options = {}, &block)
+    bulb_type = (attributes || {}).delete(:bulb_type)
+
+    if options && options[:as] == :admin && bulb_type.present?
+      bulb_class = "#{bulb_type.to_s.camelize}Bulb".constantize
+      bulb_class.new(attributes, options, &block)
+    else
+      super
+    end
+  end
 end
+
+class CustomBulb < Bulb
+end