fix: Polymorphic + STI uniqueness check

Instead of just initializing a new class that inherits from the parent
class, we are initializing a new class but also overriding some
method definitions. That is necessary because the Rails guides
recommend that whenever you have a polymorphic association and
STI you should override the `..._type` column to use the type name
of the base STI class.

lib/shoulda/matchers/active_record/uniqueness/model.rb

@@ -29,7 +29,19 @@ def next
           end
 
           def symlink_to(parent)
-            namespace.set(name, Class.new(parent))
+            table_name = parent.table_name
+
+            new_class = Class.new(parent) do
+              define_singleton_method :table_name do
+                table_name
+              end
+
+              define_singleton_method :base_class do
+                self
+              end
+            end
+
+            namespace.set(name, new_class)
           end
 
           def to_s

spec/unit/shoulda/matchers/active_record/validate_uniqueness_of_matcher_spec.rb

@@ -1274,6 +1274,36 @@ def configure_validation_matcher(matcher)
         scoped_to(:favoriteable_type)
     end
 
+    context 'when polymorphic model is used on a STI model' do
+      it 'still works' do
+        columns = {
+          attachable_id: { type: :integer, options: { null: false } },
+          attachable_type: { type: :string, options: { null: false } },
+          title: { type: :string, options: { null: false } },
+        }
+
+        asset_model = define_model 'Asset', columns do
+          belongs_to :attachable, polymorphic: true
+
+          validates_uniqueness_of :title, scope: [:attachable_id, :attachable_type]
+
+          def attachable_type=(class_name)
+            super(class_name.constantize.base_class.to_s)
+          end
+        end
+
+        post_model = define_model 'Post', {} do
+          has_many :assets, as: :attachable, dependent: :destroy
+        end
+
+        guest_post_model = define_model 'GuestPost', {}, parent_class: post_model, table_name: 'posts'
+
+        asset = asset_model.create!(attachable: guest_post_model.create!, title: 'foo')
+
+        expect(asset).to validate_uniqueness_of(:title).scoped_to(:attachable_id, :attachable_type)
+      end
+    end
+
     context 'if the model the *_type column refers to is namespaced, and shares the last part of its name with an existing model' do
       it 'still works' do
         define_class 'User'