Allow to use frozen scope

CHANGELOG.md

@@ -9,6 +9,7 @@
 
 ### Bug fixes
 
+* [#315](https://github.com/rubocop-hq/rubocop-rails/pull/315): Allow to use frozen scope for `Rails/UniqueValidationWithoutIndex`. ([@krim][])
 * [#313](https://github.com/rubocop-hq/rubocop-rails/pull/313): Fix `Rails/ActiveRecordCallbacksOrder` to preserve the original callback execution order. ([@eugeneius][])
 
 ### Changes
@@ -256,3 +257,4 @@
 [@jcoyne]: https://github.com/jcoyne
 [@fatkodima]: https://github.com/fatkodima
 [@taylorthurlow]: https://github.com/taylorthurlow
+[@krim]: https://github.com/krim

lib/rubocop/cop/rails/unique_validation_without_index.rb

@@ -97,6 +97,8 @@ def column_names_from_scope(node)
           scope = find_scope(uniq)
           return unless scope
 
+          scope = unfreeze_scope(scope)
+
           case scope.type
           when :sym, :str
             [scope.value]
@@ -114,6 +116,10 @@ def find_scope(pairs)
           end
         end
 
+        def unfreeze_scope(scope)
+          scope.send_type? && scope.method?(:freeze) ? scope.children.first : scope
+        end
+
         def class_node(node)
           node.each_ancestor.find(&:class_type?)
         end

spec/rubocop/cop/rails/unique_validation_without_index_spec.rb

@@ -179,6 +179,16 @@ class WrittenArticles
             end
           RUBY
         end
+
+        context 'when scope is frozen' do
+          it 'does not register an offense' do
+            expect_no_offenses(<<~RUBY)
+              class WrittenArticles
+                validates :a_id, uniqueness: { scope: [:b_id, :c_id].freeze }
+              end
+            RUBY
+          end
+        end
       end
     end