Mongodb $lookup Not working with _id

Question

wend try with this query, return the lookup is empty

db.getCollection(\'tests\').aggregate([
    {$match: {typet:\'Req\'}},
    {$project: {incharge:1}},
            


        

Answer 1

Your lookup Query is correct. But it is trying to compare a string type (incharge) with ObjectId (_id). Convert the string to ObjectId as shown below. It works for me.

db.getCollection('tests').aggregate([
{$match: {typet:'Req'}},
{$project: {
   incharge:{
     $toObjectId:"$incharge"
   }
},
{$lookup:{
        from: "users",
        localField: "incharge", //this is the _id user from tests
        foreignField: "_id", //this is the _id from users
        as: "user"
}}