Question 116148 in Mongodb Question posted in
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MongoDB $match in a aggregate lookup not working as expected

When i run this query:

db.friendRequests.aggregate([
        $lookup: {
          from: "users",
          localField: "author",
          foreignField: "_id",
          pipeline: [
            {
              $match: {
                $expr: {
                  friend_id: new mongoose.Types.ObjectId(userid),
                },
              },
            },
          ],
          as: "userdata",
        }
])

It returns every entry in the collection, but theres a pipeline in it. Then why is it not working?

Can you help me? Thanks!

Playground:
https://mongoplayground.net/p/Eh2j8lU4IQl

Tags:

2

Answers


  1. 0

    For mongodb version under 5.0 (Thanks for the remark @user20042973):
    $lookup with localField and foreignField will ignore a pipeline. Remove them and add a let key in order to enable the pipeline.

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  2. 0

    The friend_id field is present in the friendRequests collection (source for the aggregation) not the users collection which is the target for the $lookup. Therefore that predicate should come in a $match stage that precedes the $lookup:

    db.friendRequests.aggregate([
  {
    $match: {
      "friend_id": ObjectId("636a88de3e45346191cf4257")
    }
  },
  {
    $lookup: {
      from: "users",
      localField: "author",
      foreignField: "_id",
      as: "userdata"
    }
  }
])

    See how it works in this playground example. Note that I changed inventory to users assuming that was just a typo in the collection name in the provided playground link.

    Original answer

    This syntax is incorrect:

    $match: { 
  $expr: {
    friend_id: new mongoose.Types.ObjectId(userid),
  },
}

    You should change it to either

    $match: { 
  friend_id: new mongoose.Types.ObjectId(userid),
}

    Or

    $match: { 
  $expr: {
    $eq: [ 
      "$friend_id", new mongoose.Types.ObjectId(userid)
    ]
  },
}
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