3、GROM 关联 (golang)

文章目录

属于

属于

belongs to 会与另一个模型建立一对一关系，因此声明的每一个模型实例都会”属于”另一个模型实例。

例如, 如果您的应用程序包含用户和配置文件, 并且可以将每个配置文件分配给一个用户

type User struct {
  gorm.Model
  Name string
}

// `Profile` 属于 `User`， 外键是`UserID` 
type Profile struct {
  gorm.Model
  UserID int
  User   User
  Name   string
}

外键

Foreign Key，若要定义属于关系的外键必须存在, 默认外键使用所有者的类型名称及其主键。

对于上述例子，定义一个属于 User 的模型，外键应该是 UserID。

GORM 提供了自定义外键的方法，例如：

type User struct {
  gorm.Model
  Name string
}

type Profile struct {
  gorm.Model
  Name      string
  User      User `gorm:"foreignkey:UserRefer"` // 将 UserRefer 作为外键
  UserRefer uint
}

关联外键

对于一个 belongs to 关系，GORM 通常使用所有者的主键作为外键的值，对于上面例子，外键的值是 User 的 ID。

当你关联一个 profile 到一个 user 时，GORM 将保存 user 的 ID 到 profile 的 UserID 字段。

你可以用 association_foreignkey 标签来更改它，例如：

type User struct {
  gorm.Model
  Refer string
  Name string
}

type Profile struct {
  gorm.Model
  Name      string
  User      User `gorm:"association_foreignkey:Refer"` // 将 Refer 作为关联外键
  UserRefer string
}

Belongs To 的使用

你可以使用 Related 查找 belongs to 关系。

db.Model(&user).Related(&profile)
 SELECT * FROM profiles WHERE user_id = 111; // 111 is user's ID

高级用法请参阅 关联模式

查询

查询

// 根据主键查询第一条记录
db.First(&user)
 SELECT * FROM users ORDER BY id LIMIT 1;

// 随机获取一条记录
db.Take(&user)
 SELECT * FROM users LIMIT 1;

// 根据主键查询最后一条记录
db.Last(&user)
 SELECT * FROM users ORDER BY id DESC LIMIT 1;

// 查询所有的记录
db.Find(&users)
 SELECT * FROM users;

// 查询指定的某条记录(仅当主键为整型时可用)
db.First(&user, 10)
 SELECT * FROM users WHERE id = 10;

Where 条件

普通 SQL

// 获取第一个匹配的记录
db.Where("name = ?", "jinzhu").First(&user)
 SELECT * FROM users WHERE name = 'jinzhu' limit 1;

// 获取所有匹配的记录
db.Where("name = ?", "jinzhu").Find(&users)
 SELECT * FROM users WHERE name = 'jinzhu';

// <>
db.Where("name <> ?", "jinzhu").Find(&users)
 SELECT * FROM users WHERE name <> 'jinzhu';

// IN
db.Where("name IN (?)", []string{"jinzhu", "jinzhu 2"}).Find(&users)
 SELECT * FROM users WHERE name in ('jinzhu','jinzhu 2');

// LIKE
db.Where("name LIKE ?", "%jin%").Find(&users)
 SELECT * FROM users WHERE name LIKE '%jin%';

// AND
db.Where("name = ? AND age >= ?", "jinzhu", "22").Find(&users)
 SELECT * FROM users WHERE name = 'jinzhu' AND age >= 22;

// Time
db.Where("updated_at > ?", lastWeek).Find(&users)
 SELECT * FROM users WHERE updated_at > '2000-01-01 00:00:00';

// BETWEEN
db.Where("created_at BETWEEN ? AND ?", lastWeek, today).Find(&users)
 SELECT * FROM users WHERE created_at BETWEEN '2000-01-01 00:00:00' AND '2000-01-08 00:00:00';

Struct & Map

// Struct
db.Where(&User{Name: "jinzhu", Age: 20}).First(&user)
 SELECT * FROM users WHERE name = "jinzhu" AND age = 20 ORDER BY id LIMIT 1;

// Map
db.Where(map[string]interface{}{"name": "jinzhu", "age": 20}).Find(&users)
 SELECT * FROM users WHERE name = "jinzhu" AND age = 20;

// 主键切片
db.Where([]int64{20, 21, 22}).Find(&users)
 SELECT * FROM users WHERE id IN (20, 21, 22);

提示 当通过结构体进行查询时，GORM将会只通过非零值字段查询，这意味着如果你的字段值为0，''， false 或者其他 零值时，将不会被用于构建查询条件，例如：

db.Where(&User{Name: "jinzhu", Age: 0}).Find(&users)
 SELECT * FROM users WHERE name = "jinzhu";

你可以使用指针或实现 Scanner/Valuer 接口来避免这个问题.

// 使用指针
type User struct {
  gorm.Model
  Name string
  Age  *int
}

// 使用 Scanner/Valuer
type User struct {
  gorm.Model
  Name string
  Age  sql.NullInt64  // sql.NullInt64 实现了 Scanner/Valuer 接口
}

Not 条件

作用与 Where 类似

db.Not("name", "jinzhu").First(&user)
 SELECT * FROM users WHERE name <> "jinzhu" ORDER BY id LIMIT 1;

// Not In
db.Not("name", []string{"jinzhu", "jinzhu 2"}).Find(&users)
 SELECT * FROM users WHERE name NOT IN ("jinzhu", "jinzhu 2");

// 不在主键切片中
db.Not([]int64{1,2,3}).First(&user)
 SELECT * FROM users WHERE id NOT IN (1,2,3) ORDER BY id LIMIT 1;

db.Not([]int64{}).First(&user)
 SELECT * FROM users ORDER BY id LIMIT 1;

// 普通 SQL
db.Not("name = ?", "jinzhu").First(&user)
 SELECT * FROM users WHERE NOT(name = "jinzhu") ORDER BY id LIMIT 1;

// Struct
db.Not(User{Name: "jinzhu"}).First(&user)
 SELECT * FROM users WHERE name <> "jinzhu" ORDER BY id LIMIT 1;

Or 条件

db.Where("role = ?", "admin").Or("role = ?", "super_admin").Find(&users)
 SELECT * FROM users WHERE role = 'admin' OR role = 'super_admin';

// Struct
db.Where("name = 'jinzhu'").Or(User{Name: "jinzhu 2"}).Find(&users)
 SELECT * FROM users WHERE name = 'jinzhu' OR name = 'jinzhu 2';

// Map
db.Where("name = 'jinzhu'").Or(map[string]interface{}{"name": "jinzhu 2"}).Find(&users)
 SELECT * FROM users WHERE name = 'jinzhu' OR name = 'jinzhu 2';

Inline Condition 内联条件

作用与 Where 类似

当内联条件与 多个立即执行方法 一起使用时, 内联条件不会传递给后面的立即执行方法。

// 通过主键获取 (只适用于整数主键)
db.First(&user, 23)
 SELECT * FROM users WHERE id = 23;
// 如果是一个非整数类型，则通过主键获取
db.First(&user, "id = ?", "string_primary_key")
 SELECT * FROM users WHERE id = 'string_primary_key';

// Plain SQL
db.Find(&user, "name = ?", "jinzhu")
 SELECT * FROM users WHERE name = "jinzhu";

db.Find(&users, "name <> ? AND age > ?", "jinzhu", 20)
 SELECT * FROM users WHERE name <> "jinzhu" AND age > 20;

// Struct
db.Find(&users, User{Age: 20})
 SELECT * FROM users WHERE age = 20;

// Map
db.Find(&users, map[string]interface{}{"age": 20})
 SELECT * FROM users WHERE age = 20;

Extra Querying option 其它查询选项

// 为查询 SQL 添加额外的 SQL 操作
db.Set("gorm:query_option", "FOR UPDATE").First(&user, 10)
 SELECT * FROM users WHERE id = 10 FOR UPDATE;

FirstOrInit

获取匹配的第一条记录，否则根据给定的条件初始化一个新的对象 (仅支持 struct 和 map 条件)

// 未找到
db.FirstOrInit(&user, User{Name: "non_existing"})
 user -> User{Name: "non_existing"}

// 找到
db.Where(User{Name: "Jinzhu"}).FirstOrInit(&user)
 user -> User{Id: 111, Name: "Jinzhu", Age: 20}
db.FirstOrInit(&user, map[string]interface{}{"name": "jinzhu"})
 user -> User{Id: 111, Name: "Jinzhu", Age: 20}

Attrs

如果记录未找到，将使用参数初始化 struct.

// 未找到
db.Where(User{Name: "non_existing"}).Attrs(User{Age: 20}).FirstOrInit(&user)
 SELECT * FROM USERS WHERE name = 'non_existing' ORDER BY id LIMIT 1;
 user -> User{Name: "non_existing", Age: 20}

db.Where(User{Name: "non_existing"}).Attrs("age", 20).FirstOrInit(&user)
 SELECT * FROM USERS WHERE name = 'non_existing' ORDER BY id LIMIT 1;
 user -> User{Name: "non_existing", Age: 20}

// 找到
db.Where(User{Name: "Jinzhu"}).Attrs(User{Age: 30}).FirstOrInit(&user)
 SELECT * FROM USERS WHERE name = jinzhu' ORDER BY id LIMIT 1;
 user -> User{Id: 111, Name: "Jinzhu", Age: 20}

Assign

不管记录是否找到，都将参数赋值给 struct.

// 未找到
db.Where(User{Name: "non_existing"}).Assign(User{Age: 20}).FirstOrInit(&user)
 user -> User{Name: "non_existing", Age: 20}

// 找到
db.Where(User{Name: "Jinzhu"}).Assign(User{Age: 30}).FirstOrInit(&user)
 SELECT * FROM USERS WHERE name = jinzhu' ORDER BY id LIMIT 1;
 user -> User{Id: 111, Name: "Jinzhu", Age: 30}

FirstOrCreate

获取匹配的第一条记录, 否则根据给定的条件创建一个新的记录 (仅支持 struct 和 map 条件)

// 未找到
db.FirstOrCreate(&user, User{Name: "non_existing"})
 INSERT INTO "users" (name) VALUES ("non_existing");
 user -> User{Id: 112, Name: "non_existing"}

// 找到
db.Where(User{Name: "Jinzhu"}).FirstOrCreate(&user)
 user -> User{Id: 111, Name: "Jinzhu"}

Attrs

如果记录未找到，将使用参数创建 struct 和记录.

// 未找到
db.Where(User{Name: "non_existing"}).Attrs(User{Age: 20}).FirstOrCreate(&user)
 SELECT * FROM users WHERE name = 'non_existing' ORDER BY id LIMIT 1;
 INSERT INTO "users" (name, age) VALUES ("non_existing", 20);
 user -> User{Id: 112, Name: "non_existing", Age: 20}

// 找到
db.Where(User{Name: "jinzhu"}).Attrs(User{Age: 30}).FirstOrCreate(&user)
 SELECT * FROM users WHERE name = 'jinzhu' ORDER BY id LIMIT 1;
 user -> User{Id: 111, Name: "jinzhu", Age: 20}

Assign

不管记录是否找到，都将参数赋值给 struct 并保存至数据库.

// 未找到
db.Where(User{Name: "non_existing"}).Assign(User{Age: 20}).FirstOrCreate(&user)
 SELECT * FROM users WHERE name = 'non_existing' ORDER BY id LIMIT 1;
 INSERT INTO "users" (name, age) VALUES ("non_existing", 20);
 user -> User{Id: 112, Name: "non_existing", Age: 20}

// 找到
db.Where(User{Name: "jinzhu"}).Assign(User{Age: 30}).FirstOrCreate(&user)
 SELECT * FROM users WHERE name = 'jinzhu' ORDER BY id LIMIT 1;
 UPDATE users SET age=30 WHERE id = 111;
 user -> User{Id: 111, Name: "jinzhu", Age: 30}

Advanced Query 高级查询

SubQuery 子查询

基于 *gorm.expr 的子查询

db.Where("amount > ?", db.Table("orders").Select("AVG(amount)").Where("state = ?", "paid").SubQuery()).Find(&orders)
// SELECT * FROM "orders"  WHERE "orders"."deleted_at" IS NULL AND (amount > (SELECT AVG(amount) FROM "orders"  WHERE (state = 'paid')));

选择字段

Select，指定你想从数据库中检索出的字段，默认会选择全部字段。

db.Select("name, age").Find(&users)
 SELECT name, age FROM users;

db.Select([]string{"name", "age"}).Find(&users)
 SELECT name, age FROM users;

db.Table("users").Select("COALESCE(age,?)", 42).Rows()
 SELECT COALESCE(age,'42') FROM users;

排序

Order，指定从数据库中检索出记录的顺序。设置第二个参数 reorder 为 true ，可以覆盖前面定义的排序条件。

db.Order("age desc, name").Find(&users)
 SELECT * FROM users ORDER BY age desc, name;

// 多字段排序
db.Order("age desc").Order("name").Find(&users)
 SELECT * FROM users ORDER BY age desc, name;

// 覆盖排序
db.Order("age desc").Find(&users1).Order("age", true).Find(&users2)
 SELECT * FROM users ORDER BY age desc; (users1)
 SELECT * FROM users ORDER BY age; (users2)

数量

Limit，指定从数据库检索出的最大记录数。

db.Limit(3).Find(&users)
 SELECT * FROM users LIMIT 3;

// -1 取消 Limit 条件
db.Limit(10).Find(&users1).Limit(-1).Find(&users2)
 SELECT * FROM users LIMIT 10; (users1)
 SELECT * FROM users; (users2)

偏移

Offset，指定开始返回记录前要跳过的记录数。

db.Offset(3).Find(&users)
 SELECT * FROM users OFFSET 3;

// -1 取消 Offset 条件
db.Offset(10).Find(&users1).Offset(-1).Find(&users2)
 SELECT * FROM users OFFSET 10; (users1)
 SELECT * FROM users; (users2)

总数

Count，该 model 能获取的记录总数。

db.Where("name = ?", "jinzhu").Or("name = ?", "jinzhu 2").Find(&users).Count(&count)
 SELECT * from USERS WHERE name = 'jinzhu' OR name = 'jinzhu 2'; (users)
 SELECT count(*) FROM users WHERE name = 'jinzhu' OR name = 'jinzhu 2'; (count)

db.Model(&User{}).Where("name = ?", "jinzhu").Count(&count)
 SELECT count(*) FROM users WHERE name = 'jinzhu'; (count)

db.Table("deleted_users").Count(&count)
 SELECT count(*) FROM deleted_users;

db.Table("deleted_users").Select("count(distinct(name))").Count(&count)
 SELECT count( distinct(name) ) FROM deleted_users; (count)

注意 Count 必须是链式查询的最后一个操作 ，因为它会覆盖前面的 SELECT，但如果里面使用了 count 时不会覆盖

Group & Having

rows, err := db.Table("orders").Select("date(created_at) as date, sum(amount) as total").Group("date(created_at)").Rows()
for rows.Next() {
  ...
}

rows, err := db.Table("orders").Select("date(created_at) as date, sum(amount) as total").Group("date(created_at)").Having("sum(amount) > ?", 100).Rows()
for rows.Next() {
  ...
}

type Result struct {
  Date  time.Time
  Total int64
}
db.Table("orders").Select("date(created_at) as date, sum(amount) as total").Group("date(created_at)").Having("sum(amount) > ?", 100).Scan(&results)

连接

Joins，指定连接条件

rows, err := db.Table("users").Select("users.name, emails.email").Joins("left join emails on emails.user_id = users.id").Rows()
for rows.Next() {
  ...
}

db.Table("users").Select("users.name, emails.email").Joins("left join emails on emails.user_id = users.id").Scan(&results)

// 多连接及参数
db.Joins("JOIN emails ON emails.user_id = users.id AND emails.email = ?", "[email protected]").Joins("JOIN credit_cards ON credit_cards.user_id = users.id").Where("credit_cards.number = ?", "411111111111").Find(&user)

Pluck

Pluck，查询 model 中的一个列作为切片，如果您想要查询多个列，您应该使用 Scan

var ages []int64
db.Find(&users).Pluck("age", &ages)

var names []string
db.Model(&User{}).Pluck("name", &names)

db.Table("deleted_users").Pluck("name", &names)

// 想查询多个字段？ 这样做：
db.Select("name, age").Find(&users)

扫描

Scan，扫描结果至一个 struct.

type Result struct {
  Name string
  Age  int
}

var result Result
db.Table("users").Select("name, age").Where("name = ?", "Antonio").Scan(&result)

// 原生 SQL
db.Raw("SELECT name, age FROM users WHERE name = ?", "Antonio").Scan(&result)

Has Many

Has Many

在一个 has many 关联中，其也与另一个 model 建立了一对多关系，不同于 has one，model 的拥有者可以有零个或多个实例。

例如，你的应用包含了用户和信用卡，并且每个用户可以有多张信用卡。

// User 可以有多张信用卡(CreditCards), UserID 是外键
type User struct {
  gorm.Model
  CreditCards []CreditCard
}

type CreditCard struct {
  gorm.Model
  Number   string
  UserID  uint
}

外键

Foreign Key，在 has many 关系中，被拥有 model 必须存在一个外键字段，默认的外键字段名称通常使用其拥有者 model 加上它的主键（比如 UserID, CardID, 等）。

例如：定义一个属于 User 的 model，它的外键应该为 UserID.

要使用另一个字段作为外键，你可以通过标签 foreignkey 来定制它，例如：

type User struct {
  gorm.Model
  CreditCards []CreditCard `gorm:"foreignkey:UserRefer"`
}

type CreditCard struct {
  gorm.Model
  Number    string
  UserRefer uint
}

关联外键

Association ForeignKey，GORM 通常使用拥有者的主键作为外键的值，在上面的例子中，它是 User 的 ID.

当你为用户关联信用卡时，GORM 会保存用户的 ID 到信用卡的 UserID 字段。

您可以通过标签 association_foreignkey 来改变它，例如：

type User struct {
  gorm.Model
  MemberNumber string
  CreditCards  []CreditCard `gorm:"foreignkey:UserMemberNumber;association_foreignkey:MemberNumber"`
}

type CreditCard struct {
  gorm.Model
  Number           string
  UserMemberNumber string
}

多态关联

Polymorphism Association，GORM 支持 has many 和 has one 的多态关联。

type Cat struct {
  ID    int
  Name  string
  Toy   []Toy `gorm:"polymorphic:Owner;"`
}

type Dog struct {
  ID   int
  Name string
  Toy  []Toy `gorm:"polymorphic:Owner;"`
}

type Toy struct {
  ID        int
  Name      string
  OwnerID   int
  OwnerType string
}

注意：many-to-many 明确的不支持多态关联，如果使用会抛出错误。

Has Many 的使用

你可以通过 Related 使用 has many 关联。

db.Model(&user).Related(&emails)
 SELECT * FROM emails WHERE user_id = 111; // 111 是 user 的主键

高级用法请参阅 关联模式

Many To Many

Many To Many

Many to Many 在两个 model 中添加一张连接表。

比如说，你的应用包含 User 和 Language，一个 User 可以说多种 Language，多个 User 也可以说一种 Language。

// User 拥有并属于多种 Language，使用 `user_languages` 连接表
type User struct {
  gorm.Model
  Languages         []Language `gorm:"many2many:user_languages;"`
}

type Language struct {
  gorm.Model
  Name string
}

互引用关联

// User 拥有并属于多种 Language，使用 `user_languages` 连接表
type User struct {
  gorm.Model
  Languages         []*Language `gorm:"many2many:user_languages;"`
}

type Language struct {
  gorm.Model
  Name string
  Users               []*User     `gorm:"many2many:user_languages;"`
}

var users []User
language := Language{}

db.First(&language, "id = ?", 111)

db.Model(&language).Related(&users,  "Users")
 SELECT * FROM "users" INNER JOIN "user_languages" ON "user_languages"."user_id" = "users"."id" WHERE  ("user_languages"."language_id" IN ('111'))

多外键

type CustomizePerson struct {
  IdPerson string             `gorm:"primary_key:true"`
  Accounts []CustomizeAccount `gorm:"many2many:PersonAccount;association_foreignkey:idAccount;foreignkey:idPerson"`
}

type CustomizeAccount struct {
  IdAccount string `gorm:"primary_key:true"`
  Name      string
}

Foreign Keys，它将为这两个 struct 创建多对多关系，并且他们的关系将被保存到连接表 PersonAccount ，连接表的外键为 customize_person_id_person 和 customize_account_id_account.

连接表外键

Jointable ForeignKey，如果你想改变连接表的外键，你可以使用标签 association_jointable_foreignkey 和 jointable_foreignkey.

type CustomizePerson struct {
  IdPerson string             `gorm:"primary_key:true"`
  Accounts []CustomizeAccount `gorm:"many2many:PersonAccount;foreignkey:idPerson;association_foreignkey:idAccount;association_jointable_foreignkey:account_id;jointable_foreignkey:person_id;"`
}

type CustomizeAccount struct {
  IdAccount string `gorm:"primary_key:true"`
  Name      string
}

自引用关联

Self-Referencing，在自引用的多对多关系中，你必须在连接表中修改关联外键。

使用属性名及其主键生成关联外键，使得关联外键与外键不同，比如：

type User struct {
  gorm.Model
  Friends []*User `gorm:"many2many:friendships;association_jointable_foreignkey:friend_id"`
}

GORM 会生成一个关联表，其外键为 user_id 和 friend_id，并用其保存自引用用户关系。

然后你还是可以像正常关系一样操作它们，比如：

DB.Preload("Friends").First(&user, "id = ?", 1)

DB.Model(&user).Association("Friends").Append(&User{Name: "friend1"}, &User{Name: "friend2"})

DB.Model(&user).Association("Friends").Delete(&User{Name: "friend2"})

DB.Model(&user).Association("Friends").Replace(&User{Name: "new friend"})

DB.Model(&user).Association("Friends").Clear()

DB.Model(&user).Association("Friends").Count()

Many To Many 的使用

db.Model(&user).Related(&languages, "Languages")
 SELECT * FROM "languages" INNER JOIN "user_languages" ON "user_languages"."language_id" = "languages"."id" WHERE "user_languages"."user_id" = 111

// 查询 user 时会预加载 Languages
db.Preload("Languages").First(&user)

高级用法请参阅 关联模式

关联

自动创建/更新

创建/更新记录时, GORM 将自动保存关联及其引用。如果关联具有主键, GORM 将调用 Update 来保存它, 否则将创建它。

user := User{
  Name:            "jinzhu",
  BillingAddress:  Address{Address1: "Billing Address - Address 1"},
  ShippingAddress: Address{Address1: "Shipping Address - Address 1"},
  Emails:          []Email{
    {Email: "[email protected]"},
    {Email: "[email protected]"},
  },
  Languages:       []Language{
    {Name: "ZH"},
    {Name: "EN"},
  },
}

db.Create(&user)
 BEGIN TRANSACTION;
 INSERT INTO "addresses" (address1) VALUES ("Billing Address - Address 1");
 INSERT INTO "addresses" (address1) VALUES ("Shipping Address - Address 1");
 INSERT INTO "users" (name,billing_address_id,shipping_address_id) VALUES ("jinzhu", 1, 2);
 INSERT INTO "emails" (user_id,email) VALUES (111, "[email protected]");
 INSERT INTO "emails" (user_id,email) VALUES (111, "[email protected]");
 INSERT INTO "languages" ("name") VALUES ('ZH');
 INSERT INTO user_languages ("user_id","language_id") VALUES (111, 1);
 INSERT INTO "languages" ("name") VALUES ('EN');
 INSERT INTO user_languages ("user_id","language_id") VALUES (111, 2);
 COMMIT;

db.Save(&user)

跳过自动更新

如果数据库中已存在关联, 你可能不希望对其进行更新。

可以使用 DB 设置, 将 gorm: association_autoupdate 设置为 false

// Don't update associations having primary key, but will save reference
db.Set("gorm:association_autoupdate", false).Create(&user)
db.Set("gorm:association_autoupdate", false).Save(&user)

或者使用 GORM tags gorm:"association_autoupdate:false"

type User struct {
  gorm.Model
  Name       string
  CompanyID  uint
  // Don't update associations having primary key, but will save reference
  Company    Company `gorm:"association_autoupdate:false"`
}

跳过自动创建

即使你禁用了 AutoUpdating，没有主键的关联仍然会被创建，所有关联的引用也会被保存。

如果你也想跳过，那么你可以通过 DB 的设置，将gorm:association_autocreate设置为false

// Don't create associations w/o primary key, WON'T save its reference
db.Set("gorm:association_autocreate", false).Create(&user)
db.Set("gorm:association_autocreate", false).Save(&user)

或使用 GORM tags GORM: "association_autocreate: false"

type User struct {
  gorm.Model
  Name       string
  // Don't create associations w/o primary key, WON'T save its reference
  Company1   Company `gorm:"association_autocreate:false"`
}

跳过自动创建及更新

若要禁用 自动创建 及 自动更新, 可以将这两个设置一起使用

db.Set("gorm:association_autoupdate", false).Set("gorm:association_autocreate", false).Create(&user)

type User struct {
  gorm.Model
  Name    string
  Company Company `gorm:"association_autoupdate:false;association_autocreate:false"`
}

或使用 GORM Tag gorm: save_associations

db.Set("gorm:save_associations", false).Create(&user)
db.Set("gorm:save_associations", false).Save(&user)

type User struct {
  gorm.Model
  Name    string
  Company Company `gorm:"save_associations:false:false"`
}

跳过引用的保存

如果你不想保存关联的引用，那么你可以使用下面的技巧

db.Set("gorm:association_save_reference", false).Save(&user)
db.Set("gorm:association_save_reference", false).Create(&user)

或者使用 GORM Tag

type User struct {
  gorm.Model
  Name       string
  CompanyID  uint
  Company    Company `gorm:"association_save_reference:false"`
}

关联模式

关联模式包含几个帮助方法，可以更方便的来管理关联

// 开始使用关联模式
var user User
db.Model(&user).Association("Languages")
// `user` 是源，必须包含主键
// `Languages` 是关系中的源的字段名
// 只有在满足上面两个条件时，关联模式才能正常工作，请注意检查错误：
// db.Model(&user).Association("Languages").Error

查找关联

查找匹配的关联

db.Model(&user).Association("Languages").Find(&languages)

添加关联

为many to many，has many添加新的关联关系代替当前的关联关系has one，belongs to

db.Model(&user).Association("Languages").Append([]Language{languageZH, languageEN})
db.Model(&user).Association("Languages").Append(Language{Name: "DE"})

替换关联

使用新关联替换当前关联

db.Model(&user).Association("Languages").Replace([]Language{languageZH, languageEN})
db.Model(&user).Association("Languages").Replace(Language{Name: "DE"}, languageEN)

删除关联

删除关联的引用，不会删除关联本身

db.Model(&user).Association("Languages").Delete([]Language{languageZH, languageEN})
db.Model(&user).Association("Languages").Delete(languageZH, languageEN)

清空关联

清空对关联的引用，不会删除关联本身

db.Model(&user).Association("Languages").Clear()

关联的数量

返回关联的数量

db.Model(&user).Association("Languages").Count()

Preloading (预加载)

预加载

// 下面的例子会用到 User 和 Order 结构体
type User struct {
  gorm.Model
  Username string
  Orders Order
}
type Order struct {
  gorm.Model
  UserID uint
  Price float64
}
// Preload 方法的参数应该是主结构体的字段名
db.Preload("Orders").Find(&users)
 SELECT * FROM users;
 SELECT * FROM orders WHERE user_id IN (1,2,3,4);

db.Preload("Orders", "state NOT IN (?)", "cancelled").Find(&users)
 SELECT * FROM users;
 SELECT * FROM orders WHERE user_id IN (1,2,3,4) AND state NOT IN ('cancelled');

db.Where("state = ?", "active").Preload("Orders", "state NOT IN (?)", "cancelled").Find(&users)
 SELECT * FROM users WHERE state = 'active';
 SELECT * FROM orders WHERE user_id IN (1,2) AND state NOT IN ('cancelled');

db.Preload("Orders").Preload("Profile").Preload("Role").Find(&users)
 SELECT * FROM users;
 SELECT * FROM orders WHERE user_id IN (1,2,3,4); // has many
 SELECT * FROM profiles WHERE user_id IN (1,2,3,4); // has one
 SELECT * FROM roles WHERE id IN (4,5,6); // belongs to

自动预加载

Gorm 默认总是会自动预加载关联记录

type User struct {
  gorm.Model
  Name       string
  CompanyID  uint
  Company    Company `gorm:"PRELOAD:false"` // 不会预加载
  Role       Role                           // 预加载
}

db.Set("gorm:auto_preload", true).Find(&users)

嵌套预加载

db.Preload("Orders.OrderItems").Find(&users)
db.Preload("Orders", "state = ?", "paid").Preload("Orders.OrderItems").Find(&users)

自定义预加载 SQL

你可以通过传入 func(db *gorm.DB) *gorm.DB 来自定义预加载，比如：

db.Preload("Orders", func(db *gorm.DB) *gorm.DB {
  return db.Order("orders.amount DESC")
}).Find(&users)
 SELECT * FROM users;
 SELECT * FROM orders WHERE user_id IN (1,2,3,4) order by orders.amount DESC;