$lookup : computed foreinField workaround? - mongodb

Question

For an existing mongo database, the link between 2 collections is done by :
collA : field collB_id
collB : field _id = ObjectId("a string value")
where collB_id is _id.valueOf()
i.e. : the value of collB_id of collA is "a string value"
but in a $lookup :
localField: "collB_id",
foreignField: _id.valueOf(),
don't work, so what can I do ?
Mongodb v3.6

Answer 1

If i understood you correctly, you have two collections where documents from first collection (collA) reference documents from second collection (collB). And the problem is that you store reference as a string value of that objectId, so you cant use $lookup to join those docs.
collA:
{
"_id" : ObjectId(...),
"collB_id" : "123456...",
...
}
collB:
{
"_id" : ObjectId("123456..."),
...
}
If you are using mongo 4.0+ you can do it with following aggregation:
db.getCollection('collA').aggregate([
{
$addFields: {
converted_collB_id: { $toObjectId: "$collB_id" }
}
},
{
$lookup: {
from: 'collB',
localField: 'converted_collB_id',
foreignField: '_id',
as: 'joined'
}
}
]);
Mongo 4.0 introduces new aggregation pipeline operators $toObjectId and $toString.
That allows you to add new field which will be an ObjectId created from the string value stored in collB_id, and use that new field as localField in lookup.
I would strongly advise you not to store ObjectIds as strings.
You already experienced the problem with $lookup.
Also, size of ObjectId is 12 bytes while its hex representation is 24 bytes (that is twice the size). You will probably want to index that field as well, so you want it to be as small as possible.
ObjectId also contains timestamp which you can get by calling getTimestamp()
Make your life easier by using native types when possible!
Hope this helps!