db={"users":[{_id:1,username:"name",bio:"bio",image:"url",}],"memes":[{_id:0,user_id:1,image:"meme1.jpg",likes:[{user_id:4}]},{_id:1,user_id:1,image:"meme2.jpg",likes:[{user_id:5},{user_id:6}]}]}

db.users.aggregate([{$lookup:{from:"memes",localField:"_id",foreignField:"user_id",as:"userDetails"}},{$addFields:{"likes":{"$sum":{"$map":{"input":"$userDetails","in":{"$size":"$$this.likes"}}}}}},{$project:{_id:0,user_id:"$_id",username:1,likes:1}}])