Serializer is an extensible interface that allows to customize how to serialize and deserialize data with databasae.

GORM provides some default serializers: json, gob, unixtime, here is a quick example of how to use it.

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\"

Register Serializer

A Serializer needs to implement how to serialize and deserialize data, so it requires to implement the the following interface

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)
}

For example, the default JSONSerializer is implemented like:

// 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)
}

And registered with the following code:

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

After registering a serializer, you can use it with the serializer tag, for example:

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

Customized Serializer Type

You can use a registered serializer with tags, you are also allowed to create a customized struct that implements the above SerializerInterface and use it as a field type directly, for example:

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"