como consultar a chave existe em mongoid ou mongo (ruby)

{"_id":{"$oid":"5ee9e17e88adc3a1d6c2a39f"},"title":"test","products":{"car":"yes hello"}}

Como posso consultar os documentos que têm o carro-chave mongo _id ou mongo? Eu tentei.

    irb(main):042:0> x = client[:materials].find({"products.car":{$exists=>true}})
=> #<Mongo::Collection::View:0x47269370736460 namespace='db.materials' @filter={"products.car"=>{nil=>true}} @options={}>
irb(main):043:0> x.count
Traceback (most recent call last):
        1: from (irb):43
BSON::InvalidKey (NilClass instances are not allowed as keys in a BSON document.)