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.

activerecord/lib/active_record/attribute_methods.rb

@@ -11,17 +11,30 @@ module ClassMethods
       # accessors, mutators and query methods.
       def define_attribute_methods
         return if attribute_methods_generated?
-        super(column_names)
-        @attribute_methods_generated = true
+
+        if base_class == self
+          super(column_names)
+          @attribute_methods_generated = true
+        else
+          base_class.define_attribute_methods
+        end
       end
 
       def attribute_methods_generated?
-        @attribute_methods_generated ||= false
+        if base_class == self
+          @attribute_methods_generated ||= false
+        else
+          base_class.attribute_methods_generated?
+        end
       end
 
       def undefine_attribute_methods(*args)
-        super
-        @attribute_methods_generated = false
+        if base_class == self
+          super
+          @attribute_methods_generated = false
+        else
+          base_class.undefine_attribute_methods(*args)
+        end
       end
 
       # Checks whether the method is defined in the model or any of its subclasses

activerecord/lib/active_record/base.rb

@@ -1332,7 +1332,7 @@ def compute_type(type_name)
         # Returns the class descending directly from ActiveRecord::Base or an
         # abstract class, if any, in the inheritance hierarchy.
         def class_of_active_record_descendant(klass)
-          if klass.superclass == Base || klass.superclass.abstract_class?
+          if klass == Base || klass.superclass == Base || klass.superclass.abstract_class?
             klass
           elsif klass.superclass.nil?
             raise ActiveRecordError, "#{name} doesn't belong in a hierarchy descending from ActiveRecord"

activerecord/test/cases/attribute_methods/read_test.rb

@@ -35,6 +35,7 @@ def self.columns_hash
           end
 
           def self.serialized_attributes; {}; end
+          def self.base_class; self; end
         end
       end
 

activerecord/test/cases/attribute_methods_test.rb

@@ -659,6 +659,22 @@ def test_list_of_serialized_attributes
     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)