func splitKey(key string, val interface{}) (field string, operator string, err error) {
	key = strings.Trim(key, " ")
	if "" == key {
		err = errSplitEmptyKey
		return
	}
	idx := strings.IndexByte(key, ' ')
	if idx == -1 {
		field = key
		operator = "="
		if isFieldContainOp(field) {
			field, operator = splitKeyNoSpace(field)
		}
		if reflect.ValueOf(val).Kind() == reflect.Slice {
			operator = "in"
		}
	} else {
		field = key[:idx]
		operator = strings.Trim(key[idx+1:], " ")
		operator = removeInnerSpace(operator)
	}
	return
}

func isFieldContainOp(field string) (contains bool) {
	if field == "" {
		return
	}
	ecof := field[len(field)-1:]
	contains = ecof == opEq || ecof == opLt || ecof == opGt
	return
}
`