Mappings should be inherited.

lib/simple_form/map_type.rb

@@ -1,7 +1,10 @@
+require 'active_support/core_ext/class/attribute'
+
 module SimpleForm
   module MapType
-    def mappings
-      @mappings ||= {}
+    def self.extended(base)
+      base.class_attribute :mappings
+      base.mappings = {}
     end
 
     def map_type(*types)

test/form_builder_test.rb

@@ -9,6 +9,12 @@ class FormBuilderTest < ActionView::TestCase
     end
   end
 
+  def with_custom_form_for(object, *args, &block)
+    with_concat_custom_form_for(object) do |f|
+      f.input(*args, &block)
+    end
+  end
+
   def with_button_for(object, *args)
     with_concat_form_for(object) do |f|
       f.button(*args)
@@ -583,4 +589,10 @@ class FormBuilderTest < ActionView::TestCase
     assert_select 'form ul', :count => 1
     assert_select 'form ul li', :count => 3
   end
+
+  # CUSTOM FORM BUILDER
+  test 'custom builder should inherit mappings' do
+    with_custom_form_for @user, :email
+    assert_select 'form input[type=email]#user_email.custom'
+  end
 end

test/support/misc_helpers.rb

@@ -28,4 +28,18 @@ module MiscHelpers
   def with_concat_form_for(object, &block)
     concat simple_form_for(object, &block)
   end
+
+  def with_concat_custom_form_for(object, &block)
+    concat custom_form_for(object, &block)
+  end
+
+  def custom_form_for(object, *args, &block)
+    simple_form_for(object, *(args << { :builder => CustomFormBuilder }), &block)
+  end
+end
+
+class CustomFormBuilder < SimpleForm::FormBuilder
+  def input(attribute_name, *args, &block)
+    super(attribute_name, *(args << { :input_html => { :class => 'custom' } }), &block)
+  end
 end