db={"user":[{"_id":ObjectId("5e2b9ab8b1dbae5124e4b635"),"name":"user1"},{"_id":ObjectId("5e2b9c29b1dbae5124e4b63b"),"name":"user2"},{"_id":ObjectId("5e2b9cd2b1dbae5124e4b641"),"name":"user3"},{"_id":ObjectId("5e2ba081b1dbae5124e4b64c"),"name":"user4"}],"business_account":[{"_id":ObjectId("5e2c5e433e8d406769286567"),"name":"account1","users":[ObjectId("5e2b9cd2b1dbae5124e4b641"),ObjectId("5e2b9c29b1dbae5124e4b63b")]},{"_id":ObjectId("4e2c5e433e8d40676928bb43"),"name":"account2","users":[ObjectId("5e2b9ab8b1dbae5124e4b635")]}]}

db.user.aggregate([{"$lookup":{"from":"business_account","let":{"id":"$_id"},"pipeline":[{"$match":{"$expr":{"$in":["$$id","$users"]}}},{"$project":{name:1,_id:0}}],"as":"belongs_to"}},{$unwind:{path:"$belongs_to",preserveNullAndEmptyArrays:true}},{$addFields:{belongs_to:"$belongs_to.name"}}])