cryptic mongodb error LEFT_SUBFIELD only supports Object: stats not: 6

Question

I'm having trouble figuring out what this error means

LEFT_SUBFIELD only supports Object: stats not: 6

It seems to be happening when I am inserting into my profiles collection. I am using mongoose.js. We are inserting counts of posts in each category in the stats property, e.g.

stats: {category:count, category2: count2}.

Here is my schema

var ProfileSchema = new Schema({
  uname: {
    type: String,
    required: true,
    index: true,
    unique: true
  },
  fname: String,
  lname: String,
  stats: {
    type:{},
    "default":{},
    required:true
  },
  created: {
    type:Date,
    required:true,
    "default":Date.now
  }
});

I think it might be happening when I am updating the stats object $inc counts so that stats can come out to something like this update

db.status.update({_id:xyz}, {$inc: { stats.foo : 1, stats.bar:1}})

Here's my mongoose code

      var tags = ["comedy", "action", "drama"];

      //also adding the postId to the posts collection of profile
      var updateCommand = {$push: {posts: post._id}};

      var stats = {};
      for (var i = tags.length - 1; i >= 0; i--){
        stats["stats." + tags[i].toString()] = 1;
      };
      updateCommand.$inc = stats;

      Profile.update(
        {uname: uname}, 
        updateCommand,
        {safe:true, upsert:true},
        callback
      );

Answer 1

It also happens if you're trying to update a subdocument of a non-object.

> db.test.insert({_id: 10240292, object: 'some string'})
> db.test.update({_id: 10240292}, {$set: {'object.subkey': 'some string'}})
LEFT_SUBFIELD only supports Object: object not: 2

Maybe it's not your case, but it can help somebody who googles for this error.

Answer 2

You may be running into this:

https://jira.mongodb.org/browse/SERVER-2651

or

https://jira.mongodb.org/browse/SERVER-5227

Both of which are fixed in the 2.1 dev branch already but not (yet) backported to 2.0

There is a decent discussion here about a similar issue:

https://groups.google.com/forum/?fromgroups#!topic/mongodb-user/VhjhcyEdbNQ

Basically it boils down to the fact that you are likely passing an empty key as part of the update which needs to be avoided.