Add owner_username attribute to Tournament

displaying the username of the user owning the tournament

app/serializers/tournament_serializer.rb

@@ -4,4 +4,8 @@ class TournamentSerializer < SimpleTournamentSerializer
   attributes :description
   has_many :stages
   has_many :teams
+
+  attribute :owner_username do
+    object.owner.username
+  end
 end