db={"users":[{"_id":1,userField:"abc",imageVal:1},{"_id":2,userField:"xyz",imageVal:3}],"images":[{"_id":1,imageVal:1,createdAt:75},{"_id":2,imageVal:1,createdAt:50},{"_id":3,imageVal:1,createdAt:100},{"_id":4,imageField:123}]}

db.users.aggregate([{"$lookup":{"from":"images","localField":"imageVal","foreignField":"imageVal","as":"images"}},{$addFields:{images:{$sortArray:{input:"$images",sortBy:{createdAt:-1}}}}}])