Aggregate mongoDB collections

Question

I have two collections and I want to join those two collections this is my first collection:

{
    "taskId": "62925ec4e9ebd8e244287299",
    "gebruikerId": "6287cd19af1540ea55212fee",
    "fotoUrl": "<url>",
    "afgerond": false,
    "notitie": "",
    "_id": "62927baabc3b9a3603ad56aa",
    "createdAt": "2022-05-28T19:44:42.559Z",
    "updatedAt": "2022-05-28T19:44:42.559Z",
    "__v": 0
}

And I want to join the taskId of the previous collection on the _id of this collection:

{
    "_id": "62925ec4e9ebd8e244287299",
    "taskNaam": "BodyShot",
    "omschrijving": "Neem een bodyshot uit de navel van Nils",
    "punten": 100,
    "fotoUrl": "<url>",
    "locked": true,
    "createdAt": "2022-05-28T17:41:24.358Z",
    "updatedAt": "2022-05-28T17:41:24.358Z",
    "__v": 0
}

Answer 1

Simply use $lookup in an aggregation stage like this:

db.collection1.aggregate([
  {
    "$lookup": {
      "from": "collection2",
      "localField": "taskId",
      "foreignField": "_id",
      "as": "tasks"
    }
  }
])

This query compare the fields taskId in collection1 and _id in collection2, showing the join in a field called tasks .

Example here