db={"user":[{_id:"58ebf8f24d52e9ab59b5538b",store_ids:[ObjectId("58dd4bb10e2898b0057be648"),ObjectId("58ecd57d1a2f48e408ea2a30"),ObjectId("58e7a0766de7403f5118afea")]}],"store":[{_id:ObjectId("58dd4bb10e2898b0057be648"),name:"Store A"},{_id:ObjectId("58ecd57d1a2f48e408ea2a30"),name:"Store B"},{_id:ObjectId("58e7a0766de7403f5118afea"),name:"Store C"}]}

db.user.aggregate({"$lookup":{"from":"store","localField":"store_ids","foreignField":"_id","as":"stores"}},{"$project":{store_ids:0}})