10 files changed, 129 insertions, 1 deletions
diff --git a/actionpack/CHANGELOG.md b/actionpack/CHANGELOG.md
index 4ab0857a66..a5e551c78e 100644
--- a/actionpack/CHANGELOG.md
+++ b/actionpack/CHANGELOG.md
@@ -1,3 +1,11 @@
+*   Add ability to override default form builder for a controller.
+
+        class AdminController < ApplicationController
+          default_form_builder AdminFormBuilder
+        end
+
+    *Kevin McPhillips*
+
 *   For actions with no corresponding templates, render `head :no_content`
     instead of raising an error. This allows for slimmer API controller
     methods that simply work, without needing further instructions.
diff --git a/actionpack/lib/action_controller.rb b/actionpack/lib/action_controller.rb
index 7667e469d3..a1893ce920 100644
--- a/actionpack/lib/action_controller.rb
+++ b/actionpack/lib/action_controller.rb
@@ -12,6 +12,7 @@ module ActionController
   autoload :Metal
   autoload :Middleware
   autoload :Renderer
+  autoload :FormBuilder
 
   autoload_under "metal" do
     autoload :Compatibility
diff --git a/actionpack/lib/action_controller/base.rb b/actionpack/lib/action_controller/base.rb
index e6038396f9..bfae372f53 100644
--- a/actionpack/lib/action_controller/base.rb
+++ b/actionpack/lib/action_controller/base.rb
@@ -221,6 +221,7 @@ module ActionController
 
       Cookies,
       Flash,
+      FormBuilder,
       RequestForgeryProtection,
       ForceSSL,
       Streaming,
diff --git a/actionpack/lib/action_controller/form_builder.rb b/actionpack/lib/action_controller/form_builder.rb
new file mode 100644
index 0000000000..1dce4f7ef4
--- /dev/null
+++ b/actionpack/lib/action_controller/form_builder.rb
@@ -0,0 +1,48 @@
+module ActionController
+  # Override the default form builder for all views rendered by this
+  # controller and any of its descendents. Accepts a sublcass of
+  # +ActionView::Helpers::FormBuilder+.
+  #
+  # For example, given a form builder:
+  #
+  #   class AdminFormBuilder < ActionView::Helpers::FormBuilder
+  #     def special_field(name)
+  #     end
+  #   end
+  #
+  # The controller specifies a form builder as its default:
+  #
+  #   class AdminAreaController < ApplicationController
+  #     default_form_builder AdminFormBuilder
+  #   end
+  #
+  # Then in the view any form using +form_for+ will be an instance of the
+  # specified form builder:
+  #
+  #   <%= form_for(@instance) do |builder| %>
+  #     <%= builder.special_field(:name) %>
+  #   <%= end %>
+  module FormBuilder
+    extend ActiveSupport::Concern
+
+    included do
+      class_attribute :_default_form_builder, instance_accessor: false
+    end
+
+    module ClassMethods
+      # Set the form builder to be used as the default for all forms
+      # in the views rendered by this controller and its subclasses.
+      #
+      # ==== Parameters
+      # * <tt>builder</tt> - Default form builder, an instance of +ActionView::Helpers::FormBuilder+
+      def default_form_builder(builder)
+        self._default_form_builder = builder
+      end
+    end
+
+    # Default form builder for the controller
+    def default_form_builder
+      self.class._default_form_builder
+    end
+  end
+end
diff --git a/actionpack/test/controller/form_builder_test.rb b/actionpack/test/controller/form_builder_test.rb
new file mode 100644
index 0000000000..99eeaf9ab6
--- /dev/null
+++ b/actionpack/test/controller/form_builder_test.rb
@@ -0,0 +1,17 @@
+require 'abstract_unit'
+
+class FormBuilderController < ActionController::Base
+  class SpecializedFormBuilder < ActionView::Helpers::FormBuilder ; end
+
+  default_form_builder SpecializedFormBuilder
+end
+
+class ControllerFormBuilderTest < ActiveSupport::TestCase
+  setup do
+    @controller = FormBuilderController.new
+  end
+
+  def test_default_form_builder_assigned
+    assert_equal FormBuilderController::SpecializedFormBuilder, @controller.default_form_builder
+  end
+end
diff --git a/actionview/CHANGELOG.md b/actionview/CHANGELOG.md
index 80aacf7234..74dbff80da 100644
--- a/actionview/CHANGELOG.md
+++ b/actionview/CHANGELOG.md
@@ -1,3 +1,8 @@
+*   Load the `default_form_builder` from the controller on initialization, which overrides
+    the global config if it is present.
+
+    *Kevin McPhillips*
+
 *   Accept lambda as `child_index` option in `fields_for` method.
 
     *Karol Galanciak*
diff --git a/actionview/lib/action_view/helpers/controller_helper.rb b/actionview/lib/action_view/helpers/controller_helper.rb
index 74ef25f7c1..3569fba8c6 100644
--- a/actionview/lib/action_view/helpers/controller_helper.rb
+++ b/actionview/lib/action_view/helpers/controller_helper.rb
@@ -14,6 +14,7 @@ module ActionView
         if @_controller = controller
           @_request = controller.request if controller.respond_to?(:request)
           @_config  = controller.config.inheritable_copy if controller.respond_to?(:config)
+          @_default_form_builder = controller.default_form_builder if controller.respond_to?(:default_form_builder)
         end
       end
 
diff --git a/actionview/lib/action_view/helpers/form_helper.rb b/actionview/lib/action_view/helpers/form_helper.rb
index ece117b547..7fdeca5ea8 100644
--- a/actionview/lib/action_view/helpers/form_helper.rb
+++ b/actionview/lib/action_view/helpers/form_helper.rb
@@ -114,6 +114,8 @@ module ActionView
       include ModelNaming
       include RecordIdentifier
 
+      attr_internal :default_form_builder
+
       # Creates a form that allows the user to create or update the attributes
       # of a specific model object.
       #
@@ -1233,7 +1235,7 @@ module ActionView
         end
 
         def default_form_builder_class
-          builder = ActionView::Base.default_form_builder
+          builder = default_form_builder || ActionView::Base.default_form_builder
           builder.respond_to?(:constantize) ? builder.constantize : builder
         end
     end
diff --git a/actionview/test/template/controller_helper_test.rb b/actionview/test/template/controller_helper_test.rb
new file mode 100644
index 0000000000..b5e94ea4f1
--- /dev/null
+++ b/actionview/test/template/controller_helper_test.rb
@@ -0,0 +1,21 @@
+require 'abstract_unit'
+
+class ControllerHelperTest < ActionView::TestCase
+  tests ActionView::Helpers::ControllerHelper
+
+  class SpecializedFormBuilder < ActionView::Helpers::FormBuilder ; end
+
+  def test_assign_controller_sets_default_form_builder
+    @controller = OpenStruct.new(default_form_builder: SpecializedFormBuilder)
+    assign_controller(@controller)
+
+    assert_equal SpecializedFormBuilder, self.default_form_builder
+  end
+
+  def test_assign_controller_skips_default_form_builder
+    @controller = OpenStruct.new
+    assign_controller(@controller)
+
+    assert_nil self.default_form_builder
+  end
+end
diff --git a/actionview/test/template/form_helper_test.rb b/actionview/test/template/form_helper_test.rb
index 5c55b154d3..b8cb5bd746 100644
--- a/actionview/test/template/form_helper_test.rb
+++ b/actionview/test/template/form_helper_test.rb
@@ -3269,6 +3269,30 @@ class FormHelperTest < ActionView::TestCase
     ActionView::Base.default_form_builder = old_default_form_builder
   end
 
+  def test_form_builder_override
+    self.default_form_builder = LabelledFormBuilder
+
+    output_buffer = fields_for(:post, @post) do |f|
+      concat f.text_field(:title)
+    end
+
+    expected = "<label for='title'>Title:</label> <input name='post[title]' type='text' id='post_title' value='Hello World' /><br/>"
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_lazy_loading_form_builder_override
+    self.default_form_builder = "FormHelperTest::LabelledFormBuilder"
+
+    output_buffer = fields_for(:post, @post) do |f|
+      concat f.text_field(:title)
+    end
+
+    expected = "<label for='title'>Title:</label> <input name='post[title]' type='text' id='post_title' value='Hello World' /><br/>"
+
+    assert_dom_equal expected, output_buffer
+  end
+
   def test_fields_for_with_labelled_builder
     output_buffer = fields_for(:post, @post, builder: LabelledFormBuilder) do |f|
       concat f.text_field(:title)