def as_json(*args)
      document = { COLLECTION => collection, ID => id }
      document.merge!(DATABASE => database) if database
      document
    end
def as_json(*args)
      document = { COLLECTION => collection, ID => id }
      document.merge!(DATABASE => database) if database
      document
    end
Get the DBRef as a JSON document

 @example Get the DBRef as a JSON hash.
   dbref.as_json

 @return [ Hash ] The max key as a JSON hash.

 @since 2.1.0