(I'm marking this minor due to the ancientness of the release not severity of the issue.)

We created a new index and suddenly queries using that index were reporting non-zero counts when they had zero elements in their resultset. I've put the console output in the attachment which shows cursor.count() returning non-zero but attempts to access element 0 generating an error and then shows the explain which clearly shows that `n` is 0.

The index we created was

ensureIndex({'_id.tag': 1, '_id.org': 1, '_id.course': 1, 'definition.children': 1})

When we deleted the index, the code worked once again.

This is for the open sourced edx.org code base (feel free to look at the code base.py _get_raw_parent_location)