Query MongoDB for multiple ObjectIDs in Array

Question

I have a collection "users" in my Database. A user can be a member of several teams.

I am using an Array to reference the team IDs.

{ _id: ObjectId("552544fd600135861d9e47d5)", 
  name : "User1",
  teams: [
        "5527a9493ebbe2452666c238",
        "5527b1be3371e3a827fa602c"
    ]
}

The teams are nothing more than a collection of:

{ _id: ObjectId("5527a9493ebbe2452666c238"),
  name: "Team 1"
}
{ _id: ObjectId("5527b1be3371e3a827fa602c"),
  name: "Team 2"
}

Now I want to get the names of all the teams in which the user is a member.

I have only found the way of querying it this way:

db.teams.find(
   {_id:{$in:
      [ObjectId("5527a9493ebbe2452666c238"),
      ObjectId("5527b1be3371e3a827fa602c")]
   }})

To do this, I would need to create an array specifically for this query. I would rather try to avoid this, because I already have the IDs available as an array in the string format. Something like this would be great:

db.teams.find(
   {_id:{$in:  
          ["5527a9493ebbe2452666c238",
          "5527b1be3371e3a827fa602c"]  // Strings here, not ObjectIDs
   }})

But this does not work. Is there any comfortable way of querying an ObjectID with a set of string IDs?

Thanks & regards Rolf

Hi @Rolf can you explain more and I don't know why you check in teams collection you simply check teams ids presents in users collection — Neo-coder
– Neo-coder, Commented Apr 10, 2015 at 12:11
I want to get the names of the teams (and bit more metadata, which is not illustrated here). In the Users collection I have only the TeamIDs available as a reference to the teams collection . — Rolf Beh
– Rolf Beh, Commented Apr 10, 2015 at 12:16

chridam · Accepted Answer · 2015-04-10 15:55:07Z

8

You could use a combination of mongodb's findOne() and find() cursor methods together with the native JavaScript map method to first get the team id's for a specific user (which will be a string array), then use the map function to map the teams' string id's array to an ObjectId's array, and finally query the teams collection with the resulting array as the $in operator expression:

var teams = db.users.findOne({"name": "User1"}).teams;
var obj_ids = teams.map(function (item){ return ObjectId(item)});
db.teams.find({ "_id": { "$in": obj_ids } });

Output:

/* 0 */
{
    "_id" : ObjectId("5527a9493ebbe2452666c238"),
    "name" : "Team 1"
}

/* 1 */
{
    "_id" : ObjectId("5527b1be3371e3a827fa602c"),
    "name" : "Team 2"
}

answered Apr 10, 2015 at 15:55

chridam

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1 Comment

Rolf Beh Over a year ago

That's a nice workaround! Thanks. So far I've iterated through the array "on my own", which is basically just another way to do this. But I was wondering if there is any native MongoDB Feature which can help me with that. Maybe my datamodel is just wrong, but I think it made sense to use a referencing model here.

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