code: func (t *Transaction) saveNumericIndex(mr *modelRef, fs *fieldSpec) {
  	fieldValue := mr.fieldValue(fs.name)
  	if fieldValue.Kind() == reflect.Ptr && fieldValue.IsNil() {
  		return
  	}
  	score := numericScore(fieldValue)
  	indexKey, err := mr.spec.fieldIndexKey(fs.name)
  	if err != nil {
  		t.setError(err)
  	}
  	t.Command("ZADD", redis.Args{indexKey, score, mr.model.ModelID()}, nil)
  }
docs: // saveNumericIndex adds commands to the transaction for saving a numeric
  // index on the given field.