Mongoid: how to query for all objects where value is nil?

Question

I'm having a difficult time doing something such as:

Something.where(:field => nil) 

or

Something.where(:field => { '$eq' => nil })

What's the right way to handle this in Mongoid?

Answer 1

That's the right way to do it. To find cars whose engine is nil, for example, use:

# Cars that have a _nil_ engine.
Car.where(:engine => nil)

If you're trying to look for the absence of a field (rather than one that's set to nil), use the $exists predicate:

# Cars that lack an engine entirely.
Car.where(:engine.exists => false)

Note that setting a field foo to be nil and lacking a field named foo are two different things.