For the all guys, sharing solution:
in your aggregate:
{
'$lookup': {
//searching collection name
'from': 'products',
//setting variable [searchId] where your string converted to ObjectId
'let': {"searchId": {$toObjectId: "$vehicule_id"}},
//search query with our [searchId] value
"pipeline":[
//searching [searchId] value equals your field [_id]
{"$match": {"$expr":[ {"_id": "$$searchId"}]}},
//projecting only fields you reaaly need, otherwise you will store all - huge data loads
{"$project":{"_id": 1}}
],
'as': 'productInfo'
}
},