diff --git a/activerecord/lib/active_record/attribute_methods.rb b/activerecord/lib/active_record/attribute_methods.rb
index 2c5db51f7f..40ff273846 100644
--- a/activerecord/lib/active_record/attribute_methods.rb
+++ b/activerecord/lib/active_record/attribute_methods.rb
@@ -18,7 +18,11 @@ module ActiveRecord
       # method is defined by Active Record though.
       def instance_method_already_implemented?(method_name)
         method_name = method_name.to_s
-        @_defined_class_methods         ||= ancestors.first(ancestors.index(ActiveRecord::Base)).sum([]) { |m| m.instance_methods(false) | m.private_instance_methods(false) }.map {|m| m.to_s }.to_set
+        index = ancestors.index(ActiveRecord::Base) || ancestors.length
+        @_defined_class_methods         ||= ancestors.first(index).map { |m|
+          m.instance_methods(false) | m.private_instance_methods(false)
+        }.flatten.map {|m| m.to_s }.to_set
+
         @@_defined_activerecord_methods ||= defined_activerecord_methods
         raise DangerousAttributeError, "#{method_name} is defined by ActiveRecord" if @@_defined_activerecord_methods.include?(method_name)
         @_defined_class_methods.include?(method_name)
diff --git a/activerecord/test/cases/attribute_methods/read_test.rb b/activerecord/test/cases/attribute_methods/read_test.rb
new file mode 100644
index 0000000000..3dc69ea35e
--- /dev/null
+++ b/activerecord/test/cases/attribute_methods/read_test.rb
@@ -0,0 +1,62 @@
+require "cases/helper"
+
+module ActiveRecord
+  module AttributeMethods
+    class ReadTest < ActiveRecord::TestCase
+      class FakeColumn < Struct.new(:name)
+        def type_cast_code(var)
+          var
+        end
+
+        def type; :integer; end
+      end
+
+      def setup
+        @klass = Class.new do
+          include ActiveRecord::AttributeMethods
+          include ActiveRecord::AttributeMethods::Read
+
+          def self.column_names
+            %w{ one two three }
+          end
+
+          def self.primary_key
+          end
+
+          def self.columns
+            column_names.map { FakeColumn.new(name) }
+          end
+
+          def self.columns_hash
+            puts caller
+            Hash[column_names.map { |name|
+              [name, FakeColumn.new(name)]
+            }]
+          end
+
+          def self.serialized_attributes; {}; end
+        end
+      end
+
+      def test_define_attribute_methods
+        instance = @klass.new
+
+        @klass.column_names.each do |name|
+          assert ! instance.methods.map(&:to_s).include?(name)
+        end
+
+        @klass.define_attribute_methods
+
+        @klass.column_names.each do |name|
+          assert(instance.methods.map(&:to_s).include?(name), "#{name} is not defined")
+        end
+      end
+
+      def test_attribute_methods_generated?
+        assert(!@klass.attribute_methods_generated?, 'attribute_methods_generated?')
+        @klass.define_attribute_methods
+        assert(@klass.attribute_methods_generated?, 'attribute_methods_generated?')
+      end
+    end
+  end
+end