Add more details around querying null/non-existent values

[afeld]

I've been using Mongoid for years and I still find this confusing: if you have a field that is boolean, for example, how do you query for documents where the field is either not present, null or false? I know part of this issue rests on the MongoDB query syntax being somewhat vague about how things like .find({myfield: null}) should be handled.