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Javascript – how can I find documents that aren't referenced by a document from another collection

I have two models called session and unreadcount. I need to get that particular session count from another table. below are my two Mongodb models.

var UnreadCountSchema = new mongoose.Schema({
    userId: { type: String, required: true },
    sessionId: { type: String, required: true},
    unreadCount: { type: Number, required: true, default: 0  },
    isDeleted: { type: Boolean, required: true, default: 0 },
}, { timestamps: true });

module.exports = mongoose.model("UnreadCount", UnreadCountSchema);

var SessionSchema = new mongoose.Schema({
    name: { type: String, required: false },
    subject: { type: String, required: false },
    sessionCode: { type: String, required: false },
}, { timestamps: true });
module.exports = mongoose.model("Session", SessionSchema);

I have not used referencing and relation. I need to get a count when I fetch the session. I have tried lookup it doesn’t work. suggest me a way to do this

The following is my code that i executed. count is there but iam not getting the result.

const response = await SessionModel.aggregate([
            {
                $match: query,
            },
            {
                $lookup: {
                    from: "UnreadCount",
                    localField: "_id",
                    foreignField: "sessionId",
                    as: "unreadCounts",
                },
            },
            {
                $addFields: {
                    unreadCount: {
                        $cond: {
                            if: { $gt: [{ $size: "$unreadCounts" }, 0] },
                            then: { $arrayElemAt: ["$unreadCounts.unreadCount", 0] },
                            else: 0,
                        },
                    },
                },
            },
            // Optionally sort the sessions by lastMessage createdAt
            // { $sort: { "lastMessage.createdAt": -1 } },
        ])
Tags:

2

Answers


  1. Chosen as BEST ANSWER
    0

    we have to use the model name that was created in Mongodb database and also convert the object id to a string. according to the type of data we are going to compare.

    The model name mentioned in code was unreadCount in database it was like unreadcounts.

    {
            $lookup: {
                from: "unreadcounts",
                let: { id: { $toString: "$_id" } },
                pipeline: [
                    {
                        $match: {
                            $expr: {
                                $eq: ["$sessionId", "$$id"]
                            }
                        }
                    }
                ],
                as: "unreadcounts"
            }
        }

  2. 0

    Wrote the answer from the comment as the answer post.

    Ensure that both fields must have the same type for the equality match. In this case, you can use the $lookup with pipeline to convert the sessionId from the UnreadCount collection to ObjectId.

    {
  $lookup: {
    from: "UnreadCount",
    let: {
      sessionId: "$_id"
    },
    pipeline: [
      {
        $match: {
          $expr: {
            $eq: [
              "$$sessionId",
              {
                $toObjectId: "$sessionId"
              }
            ]
          }
        }
      }
    ],
    as: "unreadCounts"
  }
}

    Or you can convert the _id from the Session collection as string type.

    {
  $lookup: {
    from: "UnreadCount",
    let: {
      sessionId: { $toString: "$_id" }
    },
    pipeline: [
      {
        $match: {
          $expr: {
            $eq: [
              "$$sessionId",
              "$sessionId"
            ]
          }
        }
      }
    ],
    as: "unreadCounts"
  }
}
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