compare string and objectId in lookup - mongodb

Question

I foundout that there is no option to compare these two .If there is any alternate please tell me.
As you see i have two collection event and eventuser i want list of user from eventuser on the basis
of event name in event collection.
But i have userId in string format then how to compare userId column
in event with _id column in eventuser collection.
Event collection
{
"_id" : ObjectId("5b5867500be60f139e67c908"),
"userId" : "5b58674e0be60f139e67cfea",
"name" : "Add to Cart",
},
{
"_id" : ObjectId("5b5867500be60f139e67c090"),
"userId" : "5b58674e0be60f139e67cfea",
"name" : "Searched",
},
{
"_id" : ObjectId("5b5867500be60f139e67c098"),
"userId" : "5b58674e0be60f139e67cacd",
"name" : "Add to Cart",
}
EventUser Collection
{
"_id":ObjectId("5b58674e0be60f139e67cfea"),
"name":"jogendra"
},
{
"_id":ObjectId("5b58674e0be60f139e67cfcv"),`
"name":"jogendra singh"
}
mmy query- it return users array as empty list
db.getCollection("event").aggregate([
{$match:{"name":"Add to Cart"}},
{$lookup:{
from:"eventuser",
localField:"userId",
foreignFiled:"_id",
as:"users"
}}
]);

Answer 1

In $lookup $lookup you can't match with string --> _id or _id --> string
Possible case is _id --> _id or string --> string
So you need change your data like this
{
"_id" : ObjectId("5b5867500be60f139e67c908"),
"userId" : ObjectId("5b58674e0be60f139e67cfea"),
"name" : "Add to Cart",
},
{
"_id" : ObjectId("5b5867500be60f139e67c090"),
"userId" : ObjectId("5b58674e0be60f139e67cfea"),
"name" : "Searched",
},
{
"_id" : ObjectId("5b5867500be60f139e67c098"),
"userId" : ObjectId("5b58674e0be60f139e67cacd"),
"name" : "Add to Cart",
}
otherwise, you need to upgrade your MongoDB version 4 and you can use $toObjectId $toObjectId
db.collection.aggregate([
{ $match: { "name": "Add to Cart" } },
{
$addFields: {
convertedId: { $toObjectId: "$userId" }
}
},
{
"$lookup": {
"from": "from_collection",
"localField": "convertedId",
"foreignField": "_id",
"as": "data"
}
}
]);