def assign!(field)
      options = self.class.incrementing_fields[field]
      fail AutoIncrementsError if options[:auto]
      fail AlreadyAssignedError if send(field).present?
      increment!(field, options)
    end
def assign!(field)
      options = self.class.incrementing_fields[field]
      fail AutoIncrementsError if options[:auto]
      fail AlreadyAssignedError if send(field).present?
      increment!(field, options)
    end
Manually assign the next number to the passed autoinc field.

 @raise [ Mongoid::Autoinc::AutoIncrementsError ] When `auto: true` is set
 in the increments call for `field`
 @raise [ AlreadyAssignedError ] When called more then once.

 @return [ Fixnum ] The assigned number