define field methods in an included module

replicates ActiveModel behaviour which allows you to override a field
method and still call super

lib/mongoid/fields.rb

@@ -50,11 +50,21 @@ def set_field(name, options = {})
 
       # Create the field accessors.
       def create_accessors(name, meth, options = {})
-        define_method(meth) { read_attribute(name) }
-        define_method("#{meth}=") { |value| write_attribute(name, value) }
-        define_method("#{meth}?") do
-          attr = read_attribute(name)
-          (options[:type] == Boolean) ? attr == true : attr.present?
+        generated_field_methods.module_eval do
+          define_method(meth) { read_attribute(name) }
+          define_method("#{meth}=") { |value| write_attribute(name, value) }
+          define_method("#{meth}?") do
+            attr = read_attribute(name)
+            (options[:type] == Boolean) ? attr == true : attr.present?
+          end
+        end
+      end
+
+      def generated_field_methods
+        @generated_field_methods ||= begin
+          mod = Module.new
+          include mod
+          mod
         end
       end
     end

spec/unit/mongoid/fields_spec.rb

@@ -86,6 +86,19 @@
         Person.new.testing?.should be_false
       end
 
+      it "adds field methods in a module to allow overriding and preserve inheritance" do
+        Person.class_eval do
+          attr_reader :testing_override_called
+          def testing=(value)
+            @testing_override_called = true
+            super
+          end
+        end
+
+        @person = Person.new
+        @person.testing = 'Test'
+        @person.testing_override_called.should be_true
+      end
     end
 
     context "when the type is an object" do