MongoDB comparison operators with null

Nitty-Gritty Details

Reading through the latest Mongo source, there's basically 2 cases when doing comparisons involving null:

1. If the canonical types of the BSON elements being compared are different, only equality comparisons (==, >=, <=) of null & undefined will return true; otherwise any comparison with null will return false.
   Note: No other BSON type has the same canonical type as null.
2. If the canonical types are the same (i.e., both elements are null), then compareElementValues is called. For null, this just returns the difference between the canonical type of both BSON elements and then carries out the requested comparison against 0.
   For example, null > null would translate into (5-5) > 0 --> False because the canonical type of null is 5.
   Similarly, null < null would translate into (5-5) < 0 --> False.

This means null can only ever be equal to null or undefined. Any other comparison involving null will always return false.

Is this a Bug?

Updated Answer:

The documentation for the comparison operators ($gt, $lt) references the documentation which you originally linked, which implies that the comparison operators should work with null. Furthermore, query sorting (i.e., db.find().sort()) does accurately follow the documented Comparison/Sort behavior.

This is, at the very least, inconsistent. I think it would be worth submitting a bug report to MongoDB's JIRA site.

Original Answer:

I don't think this behavior is a bug.

The general consensus for Javascript is that undefined means unassigned while null means assigned but otherwise undefined. Value comparisons against undefined, aside from equality, don't make sense, at least in a mathematical sense.

Given that BSON draws heavily from JavaScript, this applies to MongoDB too.