Database
db={"users":[{_id:ObjectId("5a934e000102030405000001"),name:"ABC",team:ObjectId("5a934e000102030405000000")}],"teams":[{_id:ObjectId("5a934e000102030405000000"),name:"T1",coaches:["5a934e000102030405000001"]}]}
Query
db.teams.aggregate([{$match:{coaches:"5a934e000102030405000001"}},{$addFields:{coaches:{$map:{input:"$coaches",in:{$toObjectId:"$$this"}}}}},{$lookup:{from:"users",localField:"coaches",foreignField:"_id",as:"coaches"}}])