Driver's DBRef class overrides Hash.from_bson to specially handle hashes with $ref keys and produce a DBRef in this case.

Unfortunately, DBRef is not a subclass of Hash, which can cause problems because now the type of return value of from_hash depends on the data being deserialized.

This probably should be repaired as part of common DBRef work.

Note that the override happens in the driver, not in bson-ruby.

irb(main):007:0> e={'$ref'=>'x','$id'=>123,'x'=>42}
=> {"$ref"=>"x", "$id"=>123, "x"=>42}
irb(main):011:0> Hash.from_bson(BSON::ByteBuffer.new(e.to_bson.to_s)).is_a?(Hash)
=> false