Added Cop to blacklist polymorphic associations

One should really use a separate table instead of using polymorphic
associations.

See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11168 for
more information.

4 files changed, 40 insertions, 11 deletions

diff --git a/rubocop/cop/activerecord_serialize.rb b/rubocop/cop/activerecord_serialize.rb
index bfa0cff9a67..9bdcc3b4c34 100644
--- a/rubocop/cop/activerecord_serialize.rb
+++ b/rubocop/cop/activerecord_serialize.rb
@@ -1,24 +1,18 @@
+require_relative '../model_helpers'
+
 module RuboCop
   module Cop
     # Cop that prevents the use of `serialize` in ActiveRecord models.
     class ActiverecordSerialize < RuboCop::Cop::Cop
+      include ModelHelpers
+
       MSG = 'Do not store serialized data in the database, use separate columns and/or tables instead'.freeze
 
       def on_send(node)
-        return unless in_models?(node)
+        return unless in_model?(node)
 
         add_offense(node, :selector) if node.children[1] == :serialize
       end
-
-      def models_path
-        File.join(Dir.pwd, 'app', 'models')
-      end
-
-      def in_models?(node)
-        path = node.location.expression.source_buffer.name
-
-        path.start_with?(models_path)
-      end
     end
   end
 end
diff --git a/rubocop/cop/polymorphic_associations.rb b/rubocop/cop/polymorphic_associations.rb
new file mode 100644
index 00000000000..7d554704550
--- /dev/null
+++ b/rubocop/cop/polymorphic_associations.rb
@@ -0,0 +1,23 @@
+require_relative '../model_helpers'
+
+module RuboCop
+  module Cop
+    # Cop that prevents the use of polymorphic associations
+    class PolymorphicAssociations < RuboCop::Cop::Cop
+      include ModelHelpers
+
+      MSG = 'Do not use polymorphic associations, use separate tables instead'.freeze
+
+      def on_send(node)
+        return unless in_model?(node)
+        return unless node.children[1] == :belongs_to
+
+        node.children.last.each_node(:pair) do |pair|
+          key_name = pair.children[0].children[0]
+
+          add_offense(pair, :expression) if key_name == :polymorphic
+        end
+      end
+    end
+  end
+end
diff --git a/rubocop/model_helpers.rb b/rubocop/model_helpers.rb
new file mode 100644
index 00000000000..309723dc34c
--- /dev/null
+++ b/rubocop/model_helpers.rb
@@ -0,0 +1,11 @@
+module RuboCop
+  module ModelHelpers
+    # Returns true if the given node originated from the models directory.
+    def in_model?(node)
+      path = node.location.expression.source_buffer.name
+      models_path = File.join(Dir.pwd, 'app', 'models')
+
+      path.start_with?(models_path)
+    end
+  end
+end
diff --git a/rubocop/rubocop.rb b/rubocop/rubocop.rb
index 22815090508..dae30969abf 100644
--- a/rubocop/rubocop.rb
+++ b/rubocop/rubocop.rb
@@ -2,6 +2,7 @@ require_relative 'cop/custom_error_class'
 require_relative 'cop/gem_fetcher'
 require_relative 'cop/activerecord_serialize'
 require_relative 'cop/redirect_with_status'
+require_relative 'cop/polymorphic_associations'
 require_relative 'cop/migration/add_column'
 require_relative 'cop/migration/add_column_with_default_to_large_table'
 require_relative 'cop/migration/add_concurrent_foreign_key'