如何将文本列的值指定为字符串联合？

Question

我有一个结构 Document:

type Document struct {
    ID               entity.EntityID
    Type             string
    ContentType      files.MIMEType
    URL              string
    UploadedByUserID entity.EntityID
    CreatedAt        time.Time
}

type MIMEType string

const (
    MIMETypeJPG MIMEType = "image/jpeg"
    MIMETypePDF MIMEType = "application/pdf"
    MIMETypePNG MIMEType = "image/png"
    MIMETypeTXT MIMEType = "text/plain"
)

它映射到 documents table，其中 ContentType 列的类型为 varchar(1024):

create table documents(
    id bigint not null default nextval('documents_id_seq') PRIMARY KEY,
    url varchar(1024) not null,
    type varchar(1024) not null,
    content_type varchar(1024) not null,
    uploaded_by_user_id bigint,
    created_at TIMESTAMP WITH TIME ZONE DEFAULT CURRENT_TIMESTAMP not null
);

Create(&d) 其中 d 是一个 Document 实例给我一个错误：

[2.530ms] [rows:1] INSERT INTO "documents" ("type","content_type","url","uploaded_by_user_id","created_at") VALUES ('other_freight','image/png','https://lateralline-documents-dev.s3.us-west-2.amazonaws.com/doc-other_freight-2021_08_25__11_15_13_379569000.png','253608954016301056','2021-08-25 11:15:13.82') RETURNING "id"
interface conversion: interface {} is files.MIMEType, not string

我想告诉gorm，当我读写documents.content_type字段时，它不仅仅是一个任意字符串，它应该是一个files.MIMEType值。有没有办法做到这一点？我在其他 ORM 中看到过此功能。

Answer 1

您可能想查看 custom types in go-gorm (or check this 答案）。

简而言之，您需要为 MIMEType 结构实现 Scanner 和 Valuer 接口。

func (m *MIMEType) Scan(value interface{}) error {
  val, ok := value.(string)
  if !ok {
    return errors.New(fmt.Sprint("Failed to unmarshal string value:", value))
  }
  
  *m = MIMEType(val)
  return nil
}

func (m MIMEType) Value() (driver.Value, error) {
  return string(m), nil
}

如何将文本列的值指定为字符串联合？

How to specify a text column's value as a string union?

go

go-gorm