Database
db={"user":[{_id:"5f425bdb311b791670d60de6",userid:"5f41115fbd904134883ae2d8",OverView:"sdsdssdsd",skills:[1,2],/** skill id*/Education:["5f453e7f53895727f0e39d82","5f453fb963d4ab181c115982"],location:"India",}],"skill":[{_id:"5f48d5d1b98bffee67b49917",skillid:1,name:"HTML"},{_id:"5f48d612b98bffee67b49919",skillid:2,name:"PHP"}]}
Query
db.user.aggregate([{$lookup:{from:"skill",localField:"skills",foreignField:"skillid",as:"skills"}},{$addFields:{skills:{$reduce:{input:"$skills",initialValue:[],in:{$concatArrays:[["$$this.name"],"$$value"]}}}}}])