$lookup - Match from key inside array of objects in foreign collection

Hi,

How to perform a lookup from an array of objects in a foreign collection?

Example:

user collection:

{
  "uId": "user1"
}

Match Details Collection:

{
  "matchId": 123,
  "users": [
    {
      "uId": "user1",
      "session": "active",
    },
    {
      "uId": "user2",
      "session": "active",
    },
],
}

How do I perform a lookup on Match Details Collection where the users.uId is the link between these two collections without the use of the $unwind stage inside the pipeline parameter of the $lookup stage.

I tried using the $eq operator, but it didn’t work.

{
  "$lookup": {
    "from": "matchDetails",
    "let": {"userId": "$uId"},
    pipeline: [
      {
        "$match": {
          "$expr": {
            "$eq": ["$users.uId", "$$userId"],  // <-- This doesn't work. Dont want to use `$unwind` before `$match` stage
          },
        },
      },
    ],
  }
}
db.users.aggregate([
  {
    $lookup:
      {
        from: 'matchDetails',
        localField: 'uId',
        foreignField: 'users.uId',
        as: 'details'
      }
  }
])

and this is the fix for your pipeline:

[{$lookup: {
    "from": "matchDetails",
    "let": {"userId": "$uId"},
    pipeline: [
      {
        "$match": {
          "$expr": {
            "$in": ["$$userId", "$users.uId"],  
          },
        },
      },
    ],
    as: "details"
  }}]

Notice that $users.uId is an array.

Goodluck,
Rafael,

Thanks a lot, @Rafael_Green

I taught $in will only work for arrays. But now I know that it will work even for an array of objects.

1 Like

$users.uId is array of strings

Is there any way to match inside an array of objects inside a pipeline?

I think it’s possible, my guess is that it’ll just compare the object to every object in the array of objects.

Post sample source documents and expected results.

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