Database
db={"userprofile":[{"_id":ObjectId("608108ef6cbc5641fe996e20"),"firstName":"Khansa Adilla","lastName":"Reva","mobile":"81275851188","createdOn":ISODate("2021-04-22T05:26:07.428Z"),"updatedOn":ISODate("2021-04-22T05:26:55.218Z")}],"user-wishlist":[{"_id":ObjectId("6081091f4432f9166f17c092"),"userId":"81275851188","contents":[{"_id":ObjectId("608108ef6cbc5641fe996e20"),"name":"UTBK"},{"_id":ObjectId("5fff1095f0fba6647a3c6ea7"),"name":"UTBK2"}]}]}
Query
db.userprofile.aggregate([{"$lookup":{"from":"user-wishlist","localField":"mobile","foreignField":"userId","as":"contents"}},{"$addFields":{contents:{$arrayElemAt:["$contents.contents",0]}}}])