Always generate attribute methods on the base class.

This fixes a situation I encountered where a subclass would cache the
name of a generated attribute method in @_defined_class_methods. Then,
when the superclass has it's attribute methods undefined, the subclass
would always have to dispatch through method_missing, because the
presence of the attribute in @_defined_class_methods would mean that it
is never generated again, even if undefine_attribute_methods is called
on the subclass.

There various other confusing edge cases like this. STI classes share
columns, so let's just keep all the attribute method generation state
isolated to the base class.

2 files changed, 17 insertions, 0 deletions

diff --git a/activerecord/test/cases/attribute_methods/read_test.rb b/activerecord/test/cases/attribute_methods/read_test.rb
index 3641031d12..e03ed33591 100644
--- a/activerecord/test/cases/attribute_methods/read_test.rb
+++ b/activerecord/test/cases/attribute_methods/read_test.rb
@@ -35,6 +35,7 @@ module ActiveRecord
           end
 
           def self.serialized_attributes; {}; end
+          def self.base_class; self; end
         end
       end
 
diff --git a/activerecord/test/cases/attribute_methods_test.rb b/activerecord/test/cases/attribute_methods_test.rb
index dbf5a1ba76..9815c4ba87 100644
--- a/activerecord/test/cases/attribute_methods_test.rb
+++ b/activerecord/test/cases/attribute_methods_test.rb
@@ -659,6 +659,22 @@ class AttributeMethodsTest < ActiveRecord::TestCase
     assert_equal %w(preferences), Contact.serialized_attributes.keys
   end
 
+  def test_instance_method_should_be_defined_on_the_base_class
+    subklass = Class.new(Topic)
+
+    Topic.define_attribute_methods
+
+    instance = subklass.new
+    instance.id = 5
+    assert_equal 5, instance.id
+    assert subklass.method_defined?(:id), "subklass is missing id method"
+
+    Topic.undefine_attribute_methods
+
+    assert_equal 5, instance.id
+    assert subklass.method_defined?(:id), "subklass is missing id method"
+  end
+
   private
   def cached_columns
     @cached_columns ||= (time_related_columns_on_topic + serialized_columns_on_topic).map(&:name)