db={"products":[{"_id":1,"name":"Eggs","user":1},{"_id":2,"name":"Flower","user":1},],"users":[{"_id":1,"username":"john","items":[1,2,3,4]},{"_id":2,"username":"cena","items":[1,2]},]}

db.products.aggregate([{"$lookup":{"from":"users","localField":"user","foreignField":"_id","as":"users"}},{"$unwind":"$users"},{"$project":{_id:"$_id",name:"$name","users":{itemsCount:{$size:"$users.items"}}}}])