CODEWITHSUNDEEP
node.jsmongodb
Lucdabomb
Lucdabomb

Reputation: 261

MongoDB is returning all fields instead of the matching ones

I am trying to join 2 collections with eachother with MongoDB. It does not work however the way I want.. This is what I have now:

Trying to join these 2 collections

//Group collection
_id:ObjectId(5fa2d687c59139213cf8c081)
groupname: : chatting

//Peoplegroups collection
_id:5fa2d687c59139213cf8c082
groupid:5fa2d687c59139213cf8c081
user:PNZT4lvwQZPCWql6rWVEJHjt6Pe2

Using this nodejs code I am trying to get the query:

 app.get("/groups/sync", (req, res) => {
  db.collection("groups")
    .aggregate([
      {
        $lookup: {
          from: "peoplegroups",
          localField: "groups_id",
          foreignField: "groupid",
          as: "User",
        },
      },
    ])
    .toArray(function (err, data) {
      if (err) throw res.status(500).send(err);
      res.status(201).send(data);
    });
});

Output:

[
    {
        "_id": "5fa2d687c59139213cf8c082",
        "groupid": "5fa2d687c59139213cf8c081",
        "user": "PNZT4lvwQZPCWql6rWVEJHjt6Pe2",
        "__v": 0,
        "User": [
            {
                "_id": "5fa2d687c59139213cf8c081",
                "groupname": "rsr",
                "__v": 0
            },
            {
                "_id": "5fa2d689c59139213cf8c083",
                "groupname": "rsrs",
                "__v": 0
            },
            {
                "_id": "5fa2d68ac59139213cf8c085",
                "groupname": "easa",
                "__v": 0
            }
        ]
    },

As you can see it returns all the peoplegroups fields instead of the matching groupid with the group_id. What is wrong??

Upvotes: 0

Views: 47

Answers (1)

J.F.
J.F.

Reputation: 15177

First of all your $lookup has a bad localField. Following your schema should be _id instead of groups_id.

The second thing is, what are you trying to join?

Knowing an _id get all peoplegroups which groupid is equal to _id? Or only group your peoplegroups by the same groupid?

To get the first one you need a $match stage before the $lookup like this:

db.group.aggregate([
  {
    "$match": {
      "_id": 1
    }
  },
  {
    "$lookup": {
      "from": "peoplegroups",
      "localField": "_id",
      "foreignField": "groupid",
      "as": "User"
    }
  }
])

In this way you will get all users with groupid equal to 1.

Example here

But, the second option (following your query), will group different users. Look this example to know how works.

Upvotes: 1

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