why isnt mongo returning object in array?

Question

I'm searching for documents that contain a particular _id in the contacts array in the document. So here's the structure:

{
"_id": ObjectId("505c2daea9d397f2260001cd"),
 "contacts": [
     {
       "userGroupId": ObjectId("50422b53743d6c7c0e000000"),
       "userId": ObjectId("5061f8c66096eee07d000008") 
    },
     {
       "userGroupId": ObjectId("505bf9476096ee990200000e"),
       "userId": ObjectId("505c2daea9d397f2260001ce") 
    },
      {
       "userGroupId": ObjectId("50422b75743d6c700e000004"),
       "userId": ObjectId("506cff736096ee1e26000384") 
    },
     {
       "userGroupId": ObjectId("50422b66743d6c6b0e000000"),
       "userId": ObjectId("505c2daea9d397f2260001cf") 
    },
     {
       "userGroupId": ObjectId("5050e86aa9d3977b67000000"),
       "userId": ObjectId("506494ef6096ee021f000064") 
    },
     {
       "userGroupId": ObjectId("50422b53743d6c7c0e000000"),
       "userId": ObjectId("504d72246096ee2348000008") 
    },
     {
       "userId": ObjectId("50735e8e6096ee7c510002b9"),
       "userGroupId": ObjectId("5046c73e6096ee1b77000001") 
    } 
  ]
}

Here's a second document:

{
 "_id": ObjectId("505c2da0a9d397f2260000b7"),
"contacts": [
      {
       "userGroupId": ObjectId("50422b66743d6c6b0e000000"),
       "userId": ObjectId("505c2da0a9d397f2260000b8") 
    },
     {
       "userId": ObjectId("5061f8c66096eee07d000008"),
       "userGroupId": ObjectId("50422b53743d6c7c0e000000") 
    },
     {
       "userId": ObjectId("50735e8e6096ee7c510002b9"),
       "userGroupId": ObjectId("5046c73e6096ee1b77000001") 
    } 
  ]
}

You'll notice that both documents have a userId of ObjectId("50735e8e6096ee7c510002b9") in them. I run this command:

db.collection.find({ 'contacts': 
    {$elemMatch: { userId: ObjectId("50735e8e6096ee7c510002b9") } }
});

Which should (I think) return both documents. But it only returns the second document. Not the first. I have also tried:

db.collection.find({'contacts.userId': ObjectId("50735e8e6096ee7c510002b9") });

which does the same thing as the $elemMatch query.

I'm probably missing something really elementary, but if you guys could offer some advice I'd really appreciate it.

I just realized that contacts isn't an array, it's a numerically indexed object.. (assume conversion from a php array is responsible for that) - would this be the problem? Why is it returning anything then? — Micah
– Micah, Commented Oct 18, 2012 at 19:34
The fact that contacts is a numerically keyed object instead of an array is definitely the problem here. These queries shouldn't be returning anything (and didn't when I tried them on these docs). — JohnnyHK
– JohnnyHK, Commented Oct 18, 2012 at 19:44
Correction - Rock Mongo is interpreting this incorrectly. I looked at the data from the console and it IS an array. Corrected the code above. — Micah
– Micah, Commented Oct 18, 2012 at 20:01
You're missing a } in your $elemMatch query, but other than that it's working as expected for me now, with both those queries returning both documents. — JohnnyHK
– JohnnyHK, Commented Oct 18, 2012 at 20:37
Thanks, you're right, my actual documents are much larger, so I simplified, this example is bad because it's working here, but not on my actual data. Maybe an index is corrupt or something. Thanks very much for your help. — Micah
– Micah, Commented Oct 18, 2012 at 21:17

Micah · Accepted Answer · 2012-10-18 23:53:41Z

0

This was a problem with the application layer it turns out, not Mongo. One of the records was numerically keyed, the other was not, I just had been working with Rock Mongo which seems to key all the arrays.

Thanks for your help JohnnyHK

answered Oct 18, 2012 at 23:53

Micah

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