序列化器

序列化器是一个可扩展的接口，允许自定义如何使用数据库序列化和反序列化数据。

GORM 提供了一些默认的序列化器：json、gob、unixtime，这里有一个如何使用它的快速示例。

type User struct {
  Name        []byte                 `gorm:"serializer:json"`
  Roles       Roles                  `gorm:"serializer:json"`
  Contracts   map[string]interface{} `gorm:"serializer:json"`
  JobInfo     Job                    `gorm:"type:bytes;serializer:gob"`
  CreatedTime int64                  `gorm:"serializer:unixtime;type:time"` // store int as datetime into database
}

type Roles []string

type Job struct {
  Title    string
  Location string
  IsIntern bool
}

createdAt := time.Date(2020, 1, 1, 0, 8, 0, 0, time.UTC)
data := User{
  Name:        []byte("jinzhu"),
  Roles:       []string{"admin", "owner"},
  Contracts:   map[string]interface{}{"name": "jinzhu", "age": 10},
  CreatedTime: createdAt.Unix(),
  JobInfo: Job{
    Title:    "Developer",
    Location: "NY",
    IsIntern: false,
  },
}

DB.Create(&data)
// INSERT INTO `users` (`name`,`roles`,`contracts`,`job_info`,`created_time`) VALUES
//   ("\"amluemh1\"","[\"admin\",\"owner\"]","{\"age\":10,\"name\":\"jinzhu\"}",<gob binary>,"2020-01-01 00:08:00")

var result User
DB.First(&result, "id = ?", data.ID)
// result => User{
//   Name:        []byte("jinzhu"),
//   Roles:       []string{"admin", "owner"},
//   Contracts:   map[string]interface{}{"name": "jinzhu", "age": 10},
//   CreatedTime: createdAt.Unix(),
//   JobInfo: Job{
//     Title:    "Developer",
//     Location: "NY",
//     IsIntern: false,
//   },
// }

DB.Where(User{Name: []byte("jinzhu")}).Take(&result)
// SELECT * FROM `users` WHERE `users`.`name` = "\"amluemh1\"

注册序列化器

序列化器需要实现如何序列化和反序列化数据，因此它需要实现以下接口

import "gorm.io/gorm/schema"

type SerializerInterface interface {
  Scan(ctx context.Context, field *schema.Field, dst reflect.Value, dbValue interface{}) error
  SerializerValuerInterface
}

type SerializerValuerInterface interface {
  Value(ctx context.Context, field *schema.Field, dst reflect.Value, fieldValue interface{}) (interface{}, error)
}

例如，默认的 JSONSerializer 实现如下

// JSONSerializer json serializer
type JSONSerializer struct {
}

// Scan implements serializer interface
func (JSONSerializer) Scan(ctx context.Context, field *Field, dst reflect.Value, dbValue interface{}) (err error) {
  fieldValue := reflect.New(field.FieldType)

  if dbValue != nil {
    var bytes []byte
    switch v := dbValue.(type) {
    case []byte:
      bytes = v
    case string:
      bytes = []byte(v)
    default:
      return fmt.Errorf("failed to unmarshal JSONB value: %#v", dbValue)
    }

    err = json.Unmarshal(bytes, fieldValue.Interface())
  }

  field.ReflectValueOf(ctx, dst).Set(fieldValue.Elem())
  return
}

// Value implements serializer interface
func (JSONSerializer) Value(ctx context.Context, field *Field, dst reflect.Value, fieldValue interface{}) (interface{}, error) {
  return json.Marshal(fieldValue)
}

并使用以下代码注册

schema.RegisterSerializer("json", JSONSerializer{})

注册序列化器后，您可以使用 serializer 标签来使用它，例如

type User struct {
  Name []byte `gorm:"serializer:json"`
}

自定义序列化器类型

您可以使用注册的序列化器和标签，您也可以创建一个自定义结构体，该结构体实现上述 SerializerInterface 并直接将其用作字段类型，例如

type EncryptedString string

// ctx: contains request-scoped values
// field: the field using the serializer, contains GORM settings, struct tags
// dst: current model value, `user` in the below example
// dbValue: current field's value in database
func (es *EncryptedString) Scan(ctx context.Context, field *schema.Field, dst reflect.Value, dbValue interface{}) (err error) {
  switch value := dbValue.(type) {
  case []byte:
    *es = EncryptedString(bytes.TrimPrefix(value, []byte("hello")))
  case string:
    *es = EncryptedString(strings.TrimPrefix(value, "hello"))
  default:
    return fmt.Errorf("unsupported data %#v", dbValue)
  }
  return nil
}

// ctx: contains request-scoped values
// field: the field using the serializer, contains GORM settings, struct tags
// dst: current model value, `user` in the below example
// fieldValue: current field's value of the dst
func (es EncryptedString) Value(ctx context.Context, field *schema.Field, dst reflect.Value, fieldValue interface{}) (interface{}, error) {
  return "hello" + string(es), nil
}

type User struct {
  gorm.Model
  Password EncryptedString
}

data := User{
  Password: EncryptedString("pass"),
}

DB.Create(&data)
// INSERT INTO `serializer_structs` (`password`) VALUES ("hellopass")

var result User
DB.First(&result, "id = ?", data.ID)
// result => User{
//   Password: EncryptedString("pass"),
// }

DB.Where(User{Password: EncryptedString("pass")}).Take(&result)
// SELECT * FROM `users` WHERE `users`.`password` = "hellopass"