Mongodb: query using value from a nested array

Question

I have the following schema, blog collection & friendscoll as below

blogpostcollection
    {
      "_id" : ObjectId("4fff0bf18bf0d19c4f1a5826"),
      "author" : "joe",
      "text" : "Here is the text...",
      "userid" : 0
    }
    {
      "_id" : ObjectId("4fff0bf18bf0d19c4f1a5827"),
      "author" : "blake",
      "text" : "Here is the text...",
      "userid" : 1
    }
    {
      "_id" : ObjectId("4fff0bf18bf0d19c4f1a5828"),
      "author" : "joe",
      "text" : "Here is the text...",
      "userid" : 2
    }

myfriendscoll
    {
      "myid": 999,
      "data": [
        {
          "uid": 1, 
          "name": "Raul"
        }, 
        {
          "uid": 3, 
          "name": "John K"
        } ]
    }

I want to find all documents in blogpostcollection, where the userid exists as uid, in the myfriendscoll collection. So in effect, something like..

var user = db.myfriendscoll.findOne({"myid" : 999}, {"data.uid": 1});
db.blogpostcollection.find( {"userid" : {$in : user.data.uid}});

This doesn't work, but is there a way to get it to work?...Thanks!

Asya Kamsky · Accepted Answer · 2012-07-14 22:13:47Z

1

If you are using development version 2.1 or when you move to 2.2 once it's released you can use the aggregation framework to get the format you want back from the first query:

var ret=db.myfriendscoll.aggregate([
             {$match:{"myid" : 999}},
             {$project:{_id:0,uid:"$data.uid"}}
]);
var uids=ret.result[0].uid;
db.blogpostcollection.find({userid:{$in:uids}})

answered Jul 14, 2012 at 22:13

Asya Kamsky

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Comments

narced133 · Accepted Answer · 2012-07-13 22:26:38Z

0

You'll need to extract the actual uid values into an array to use with $in. Try this:

var user = db.myfriendscoll.findOne({"myid" : 999}, {"data.uid": 1});
var uids = user.data.map(function(v){ return v.uid });
db.blogpostcollection.find( {"userid" : {$in : uids}});

answered Jul 13, 2012 at 22:26

narced133

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