Golang中gorm无法将字段更新为空值

type Student struct {
    Model
    Email       string  `form:"email" json:"email"`
    Name         string    `form:"name"  json:"name"`
}
func(c *Content) update(content Content) (err) {
    err = db.Model(&Content{}).Debug().Where("id = ?", 123).Update(&content).Error
}

func convertInterfaceToMap(values interface{}, withIgnoredField bool, db *DB) map[string]interface{} {
    var attrs = map[string]interface{}{}
    switch value := values.(type) {
    case map[string]interface{}:
        return value
    case []interface{}:
        for _, v := range value {
            for key, value := range convertInterfaceToMap(v, withIgnoredField, db) {
                attrs[key] = value
            }
        }
    case interface{}:
        reflectValue := reflect.ValueOf(values)
        switch reflectValue.Kind() {
        case reflect.Map:
            for _, key := range reflectValue.MapKeys() {
                attrs[ToColumnName(key.Interface().(string))] = reflectValue.MapIndex(key).Interface()
            }
        default:
            for _, field := range (&Scope{Value: values, db: db}).Fields() {
                if !field.IsBlank && (withIgnoredField || !field.IsIgnored) {
                    attrs[field.DBName] = field.Field.Interface()
                }
            }
        }
    }
    return attrs
}

// Fields get value's fields
func (scope *Scope) Fields() []*Field {
    if scope.fields == nil {
        var (
            fields             []*Field
            indirectScopeValue = scope.IndirectValue()
            isStruct           = indirectScopeValue.Kind() == reflect.Struct
        )
        for _, structField := range scope.GetModelStruct().StructFields {
            if isStruct {
                fieldValue := indirectScopeValue
                for _, name := range structField.Names {
                    if fieldValue.Kind() == reflect.Ptr && fieldValue.IsNil() {
                        fieldValue.Set(reflect.New(fieldValue.Type().Elem()))
                    }
                    fieldValue = reflect.Indirect(fieldValue).FieldByName(name)
                }
                //判断model中字段有没有force字段，此处使用Force，是因为解析tag时，统一转成了大写
                _, ok := structField.TagSettingsGet("FORCE")
                //如果字段为空值，且字段存在则设定为false
                fields = append(fields, &Field{StructField: structField, Field: fieldValue, IsBlank: isBlank(fieldValue) && !ok})
            } else {
                fields = append(fields, &Field{StructField: structField, IsBlank: true})
            }
        }
        scope.fields = &fields
    }
    return *scope.fields
}

type Student struct {
    Model
    Email       string  `gorm:"force" form:"email" json:"email"`
    Name         string    `gorm:"force" form:"name"  json:"name"`
}
func(c *Content) update(content Content) (err) {
    err = db.Model(&Content{}).Debug().Where("id = ?", 123).Update(&content).Error
}

func parseTagSetting(tags reflect.StructTag) map[string]string {
    setting := map[string]string{}
    for _, str := range []string{tags.Get("sql"), tags.Get("gorm")} {
        if str == "" {
            continue
        }
        tags := strings.Split(str, ";")
        for _, value := range tags {
            v := strings.Split(value, ":")
            k := strings.TrimSpace(strings.ToUpper(v[0]))
            if len(v) >= 2 {
                setting[k] = strings.Join(v[1:], ":")
            } else {
                setting[k] = k
            }
        }
    }
    return setting
}