`stored_attributes` need to be specific to a subclass.

Currently they are all stored globally in the same `Hash`.
This commit forces the creation of a per-class variable if necessary.

The behavior was exposed through the following test-case:

```
  1) Failure:
StoreTest#test_all_stored_attributes_are_returned [/Users/senny/Projects/rails/activerecord/test/cases/store_test.rb:151]:
--- expected
+++ actual
@@ -1 +1 @@
-[:color, :homepage, :favorite_food]
+[:resolution, :color, :homepage, :favorite_food]
```

3 files changed, 21 insertions, 0 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 25b905891f..ba11c7f67e 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,9 @@
+*   Fix bug where `ActiveRecord::Store` used a global `Hash` to keep track of
+    all registered `stored_attributes`. Now every subclass of
+    `ActiveRecord::Base` has it's own `Hash`.
+
+    *Yves Senn*
+
 *   Save `has_one` association when primary key is manually set.
 
     Fixes #12302.
diff --git a/activerecord/lib/active_record/store.rb b/activerecord/lib/active_record/store.rb
index a610f479f2..5fc80cdb6d 100644
--- a/activerecord/lib/active_record/store.rb
+++ b/activerecord/lib/active_record/store.rb
@@ -86,6 +86,9 @@ module ActiveRecord
           end
         end
 
+        # assign new store attribute and create new hash to ensure that each class in the hierarchy
+        # has its own hash of stored attributes.
+        self.stored_attributes = {} if self.stored_attributes.blank?
         self.stored_attributes[store_attribute] ||= []
         self.stored_attributes[store_attribute] |= keys
       end
diff --git a/activerecord/test/cases/store_test.rb b/activerecord/test/cases/store_test.rb
index c2c56abacd..0c9f7ccd55 100644
--- a/activerecord/test/cases/store_test.rb
+++ b/activerecord/test/cases/store_test.rb
@@ -150,4 +150,16 @@ class StoreTest < ActiveRecord::TestCase
   test "all stored attributes are returned" do
     assert_equal [:color, :homepage, :favorite_food], Admin::User.stored_attributes[:settings]
   end
+
+  test "stored_attributes are tracked per class" do
+    first_model = Class.new(ActiveRecord::Base) do
+      store_accessor :data, :color
+    end
+    second_model = Class.new(ActiveRecord::Base) do
+      store_accessor :data, :width, :height
+    end
+
+    assert_equal [:color], first_model.stored_attributes[:data]
+    assert_equal [:width, :height], second_model.stored_attributes[:data]
+  end
 end