As if https://jira.mongodb.org/browse/SERVER-478 were not enough, here's another one (I'm not sure if it's a Mongoid or MongoDB issue):

`
Item.where(element_ids: {'$size' => {'$eq' => 0}}).count
Item.where(element_ids: {'$size' => {'$eq' => 1}}).count
Item.where(element_ids: {'$size' => {'$eq' => 2}}).count
Item.where(element_ids: {'$size' => {'$eq' => 3}}).count
Item.where(element_ids: {'$size' => {'$ne' => 0}}).count
Item.where(element_ids: {'$size' => {'$ne' => 1}}).count
Item.where(element_ids: {'$size' => {'$ne' => 2}}).count
Item.where(element_ids: {'$size' => {'$ne' => 3}}).count
`

All these queries return exact same result, which is correct for the first query but wrong for all others.