Fixed STI classes not defining an attribute method if there is a

conflicting private method defined on its ancestors.

The problem is that `method_defined_within?(name, klass, superklass)`
only works correclty when `klass` and `superklass` are both `Class`es.

If both `klass` and `superklass` are both `Class`es, they share the
same inheritance chain, so if a method is defined on `klass` but not
`superklass`, this method must be introduced at some point between
`klass` and `superklass`.

This does not work when `superklass` is a `Module`. A `Module`'s
inheritance chain contains just itself. So if a method is defined on
`klass` but not on `superklass`, the method could still be defined
somewhere upstream, e.g. in `Object`.

This fix works by avoiding calling `method_defined_within?` with a
module while still fufilling the requirement (checking that the
method is defined withing `superclass` but not is not a generated
attribute method).

4d8ee288 is likely an attempted partial fix for this problem. This
unrolls that fix and properly check the `superclass` as intended.

Fixes #11569.

1 files changed, 19 insertions, 11 deletions

diff --git a/activerecord/test/cases/attribute_methods_test.rb b/activerecord/test/cases/attribute_methods_test.rb
index 6c581a432f..aec1ea3972 100644
--- a/activerecord/test/cases/attribute_methods_test.rb
+++ b/activerecord/test/cases/attribute_methods_test.rb
@@ -728,19 +728,27 @@ class AttributeMethodsTest < ActiveRecord::TestCase
     assert "unknown attribute: hello", error.message
   end
 
-  def test_read_attribute_overwrites_private_method_not_considered_implemented
-    # simulate a model with a db column that shares its name an inherited
-    # private method (e.g. Object#system)
-    #
-    Object.class_eval do
-      private
-      def title; "private!"; end
+  def test_global_methods_are_overwritten
+    klass = Class.new(ActiveRecord::Base) do
+      self.table_name = 'computers'
+    end
+
+    assert !klass.instance_method_already_implemented?(:system)
+    computer = klass.new
+    assert_nil computer.system
+  end
+
+  def test_global_methods_are_overwritte_when_subclassing
+    klass = Class.new(ActiveRecord::Base) { self.abstract_class = true }
+
+    subklass = Class.new(klass) do
+      self.table_name = 'computers'
     end
-    assert !@target.instance_method_already_implemented?(:title)
-    topic = @target.new
-    assert_nil topic.title
 
-    Object.send(:undef_method, :title) # remove test method from object
+    assert !klass.instance_method_already_implemented?(:system)
+    assert !subklass.instance_method_already_implemented?(:system)
+    computer = subklass.new
+    assert_nil computer.system
   end
 
   def test_instance_method_should_be_defined_on_the_base_class