Ensure uniqueness validation matcher works with STI (#1610)

The uniqueness validation matcher generates a fake class when the
scope name ends on _type. This fake class is a duplicate of the
original class. In most cases this is not a problem, however when
using single table inheritance (STI), this new fake class is not
a subclass of the original class and therefore ActiveRecord will
throw an error. See #1245

These problems are fixed by making the fake class inherrit from the
original class. Therefore it closes #1245 and closes #854

Co-authored-by: Stef Schenkelaars <[email protected]>

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

@@ -29,7 +29,7 @@ def next
           end
 
           def symlink_to(parent)
-            namespace.set(name, parent.dup)
+            namespace.set(name, Class.new(parent))
           end
 
           def to_s

spec/support/unit/model_creation_strategies/active_record.rb

@@ -74,7 +74,7 @@ def class_name
       end
 
       def table_name
-        class_name.tableize.gsub('/', '_')
+        options.fetch(:table_name, class_name.tableize.gsub('/', '_'))
       end
 
       def parent_class

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

@@ -1302,6 +1302,34 @@ def configure_validation_matcher(matcher)
           scoped_to(:favoriteable_type)
       end
     end
+
+    context 'if the model *_type column refers to an STI class' do
+      it 'still works' do
+        animal_columns = {
+          type: { type: :string, options: { null: false } },
+        }
+        animal_model = define_model 'Animal', animal_columns do
+          has_many :classifications, as: :classifiable
+        end
+        bird_model = define_model 'Bird', animal_columns, parent_class: animal_model, table_name: 'animals'
+
+        classification_columns = {
+          classifiable_id: { type: :integer },
+          classifiable_type: { type: :string },
+        }
+        classification_model = define_model 'Classification', classification_columns do
+          belongs_to :classifiable, polymorphic: true
+          validates :classifiable_id, uniqueness: { scope: :classifiable_type }
+        end
+
+        bird = bird_model.create!
+        classification = classification_model.new(classifiable: bird)
+
+        expect(classification).
+          to validate_uniqueness_of(:classifiable_id).
+          scoped_to(:classifiable_type)
+      end
+    end
   end
 
   context 'when the model does not have the attribute being tested' do