[{"id":1,"title":"Document 1","users":[{"id":"a","name":"User 1"},{"id":"b","name":"User 1"},]},{"id":2,"title":"Document 2","users":[{"id":"b","name":"User 1"},]},{"id":3,"title":"Document 3","users":[{"id":"a","name":"User 1"},{"id":"b","name":"User 1"},{"id":"c","name":"User 1"},]}]

db.collection.find({users:{$elemMatch:{id:"a"}}},{"users.$":1})