diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 881a4d2d852cc..7476b82e5f2a7 100644
--- a/activerecord/CHANGELOG.md
+++ b/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.
diff --git a/activerecord/lib/active_record/attribute_methods.rb b/activerecord/lib/active_record/attribute_methods.rb
index 9326c9c11707d..57ceec2fc50b8 100644
--- a/activerecord/lib/active_record/attribute_methods.rb
+++ b/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
 
diff --git a/activerecord/test/cases/attribute_methods_test.rb b/activerecord/test/cases/attribute_methods_test.rb
index 6c581a432fd2d..aec1ea3972e92 100644
--- a/activerecord/test/cases/attribute_methods_test.rb
+++ b/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
diff --git a/activerecord/test/fixtures/computers.yml b/activerecord/test/fixtures/computers.yml
index daf969d7da9b7..7281a4d76857e 100644
--- a/activerecord/test/fixtures/computers.yml
+++ b/activerecord/test/fixtures/computers.yml
@@ -1,4 +1,5 @@
 workstation:
   id: 1
+  system: 'Linux'
   developer: 1
   extendedWarranty: 1
diff --git a/activerecord/test/schema/schema.rb b/activerecord/test/schema/schema.rb
index 99a53434f66ae..5f2d6acbcbc52 100644
--- a/activerecord/test/schema/schema.rb
+++ b/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