如果不使用 tag 来定义 field 对应的数据库字段类型，那么系统会自动给出一个默认的字段类型，对应表如下：
| go type's kind | value method | xorm type |
| implemented Conversion | Conversion.ToDB / Conversion.FromDB | Text |
| int, int8, int16, int32, uint, uint8, uint16, uint32 | Int | |
| int64, uint64 | BigInt | |
| float32 | Float | |
| float64 | Double | |
| complex64, complex128 | json.Marshal / json.UnMarshal | Varchar(64) |
| []uint8 | Blob | |
| array, slice, map except []uint8 | json.Marshal / json.UnMarshal | Text |
| bool | 1 or 0 | Bool |
| string | Varchar(255) | |
| time.Time | DateTime | |
| cascade struct | primary key field value | BigInt |
| struct | json.Marshal / json.UnMarshal | Text |
| Others | Text |