db={"item":[{"_id":1,"k":"item1"},{"_id":2,"k":"someOtherValue"}],"user":[{"_id":1,"k2":"user1"}]}

db.item.aggregate([{$lookup:{from:"user",let:{userId:"$_id"},pipeline:[{$match:{$expr:{$eq:["$_id","$$userId"]}}}],as:"users"}}])