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).

4d8ee28 is likely an attempted partial fix for this problem. This
unrolls that fix and properly check the `superclass` as intended.

Fixes #11569.

activerecord/CHANGELOG.md

@@ -1,3 +1,10 @@
+*   Fixed STI classes not defining an attribute method if there is a
+    conflicting private method defined on its ancestors.
+
+    Fixes #11569.
+
+    *Godfrey Chan*
+
 *   Deprecate half-baked support for PostgreSQL range values with excluding beginnings.
     We currently map PostgreSQL ranges to Ruby ranges. This conversion is not fully
     possible because the Ruby range does not support excluded beginnings.

activerecord/lib/active_record/attribute_methods.rb

@@ -105,8 +105,9 @@ def instance_method_already_implemented?(method_name)
           super
         else
           # If B < A and A defines its own attribute method, then we don't want to overwrite that.
-          defined = method_defined_within?(method_name, superclass, superclass.generated_attribute_methods)
-          defined && !ActiveRecord::Base.method_defined?(method_name) || super
+          defined = method_defined_within?(method_name, superclass) &&
+            superclass.instance_method(method_name).owner != superclass.generated_attribute_methods
+          defined || super
         end
       end
 

activerecord/test/cases/attribute_methods_test.rb

@@ -728,19 +728,27 @@ def test_bulk_update_raise_unknown_attribute_errro
     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

activerecord/test/fixtures/computers.yml

@@ -1,4 +1,5 @@
 workstation:
   id: 1
+  system: 'Linux'
   developer: 1
   extendedWarranty: 1

activerecord/test/schema/schema.rb

@@ -198,6 +198,7 @@ def create_table(*args, &block)
   end
 
   create_table :computers, force: true do |t|
+    t.string :system
     t.integer :developer, null: false
     t.integer :extendedWarranty, null: false
   end