func (e *DateField) String() string {
	return e.Value().String()
}
String convert datatime to string
package orm
import (
	"fmt"
	"strconv"
	"time"
)
const (
	TypeBooleanField = 1 << iota
	TypeCharField
	TypeTextField
	TypeTimeField
	TypeDateField
	TypeDateTimeField
	TypeBitField
	TypeSmallIntegerField
	TypeIntegerField
	TypeBigIntegerField
	TypePositiveBitField
	TypePositiveSmallIntegerField
	TypePositiveIntegerField
	TypePositiveBigIntegerField
	TypeFloatField
	TypeDecimalField
	TypeJSONField
	TypeJsonbField
	RelForeignKey
	RelOneToOne
	RelManyToMany
	RelReverseOne
	RelReverseMany
)
const (
	IsIntegerField         = ^-TypePositiveBigIntegerField >> 5 << 6
	IsPositiveIntegerField = ^-TypePositiveBigIntegerField >> 9 << 10
	IsRelField             = ^-RelReverseMany >> 17 << 18
	IsFieldType            = ^-RelReverseMany<<1 + 1
)
type BooleanField bool
func (e BooleanField) Value() bool {
	return bool(e)
}
func (e *BooleanField) Set(d bool) {
	*e = BooleanField(d)
}
func (e *BooleanField) String() string {
	return strconv.FormatBool(e.Value())
}
func (e *BooleanField) FieldType() int {
	return TypeBooleanField
}
func (e *BooleanField) SetRaw(value interface{}) error {
	switch d := value.(type) {
	case bool:
		e.Set(d)
	case string:
		v, err := StrTo(d).Bool()
		if err != nil {
			e.Set(v)
		}
		return err
	default:
		return fmt.Errorf("<BooleanField.SetRaw> unknown value `%s`", value)
	}
	return nil
}
func (e *BooleanField) RawValue() interface{} {
	return e.Value()
}
var _ Fielder = new(BooleanField)
type CharField string
func (e CharField) Value() string {
	return string(e)
}
func (e *CharField) Set(d string) {
	*e = CharField(d)
}
func (e *CharField) String() string {
	return e.Value()
}
func (e *CharField) FieldType() int {
	return TypeCharField
}
func (e *CharField) SetRaw(value interface{}) error {
	switch d := value.(type) {
	case string:
		e.Set(d)
	default:
		return fmt.Errorf("<CharField.SetRaw> unknown value `%s`", value)
	}
	return nil
}
func (e *CharField) RawValue() interface{} {
	return e.Value()
}
var _ Fielder = new(CharField)
type TimeField time.Time
func (e TimeField) Value() time.Time {
	return time.Time(e)
}
func (e *TimeField) Set(d time.Time) {
	*e = TimeField(d)
}
func (e *TimeField) String() string {
	return e.Value().String()
}
func (e *TimeField) FieldType() int {
	return TypeDateField
}
func (e *TimeField) SetRaw(value interface{}) error {
	switch d := value.(type) {
	case time.Time:
		e.Set(d)
	case string:
		v, err := timeParse(d, formatTime)
		if err != nil {
			e.Set(v)
		}
		return err
	default:
		return fmt.Errorf("<TimeField.SetRaw> unknown value `%s`", value)
	}
	return nil
}
func (e *TimeField) RawValue() interface{} {
	return e.Value()
}
var _ Fielder = new(TimeField)
type DateField time.Time
func (e DateField) Value() time.Time {
	return time.Time(e)
}
func (e *DateField) Set(d time.Time) {
	*e = DateField(d)
}