$lookup to an array in MongoDB

Question

I have two collections. USERS

{
  "_id" : "myid",
  "createdAt" : ISODate("2020-12-26T14:00:11.624Z"),
  "email" : "[email protected]"
}

GROUPS

{
  "_id" : "myidgroup",
  "createdAt" : ISODate("2020-12-26T14:00:11.624Z"),
  "name" : "student"
  "userIds": ["myUserId1", "myUserId2"]
}

I want to do a $lookup to the groups collection to get all users with groups

|      email          |     groups.      |
|---------------------|------------------|
|  [email protected]  |   student        |

I expected output like this:

[
  {
     _id: "myUserId"
    email: "[email protected]",
    groups: [
              { 
                "_id": "myGroupId",
                "name": "student"
              }
            ]

  }
]

I know we can do it with new $lookup in mongodb 3.6 and above but I am using mongodb 3.4.

Any help is greatly appreciated.

what have you tried?, can you add your tries in your question. — turivishal
– turivishal, Commented Dec 26, 2020 at 15:35
I am using $match to get users, but I have no idea when I $lookup to the groups of users because in the groups userIds is an array — You Know Who
– You Know Who, Commented Dec 26, 2020 at 15:40

turivishal · Accepted Answer · 2020-12-26 15:41:15Z

1

$lookup join with groups collection, pass _id as localField and pass userIds as foreignField
$project to show required fields

db.users.aggregate([
  {
    "$lookup": {
      "from": "groups",
      "localField": "_id",
      "foreignField": "userIds",
      "as": "groups"
    }
  },
  {
    $project: {
      _id: 1,
      email: 1,
      "groups._id": 1,
      "groups.name": 1
    }
  }
])

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answered Dec 26, 2020 at 15:41

turivishal

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1 Comment

You Know Who Over a year ago

have no idea that $lookup can do this with different type of data, thanks!

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