预加载

预加载

GORM 允许在 Preload 的其它 SQL 中直接加载关系,例如:

type User struct {
gorm.Model
Username string
Orders []Order
}

type Order struct {
gorm.Model
UserID uint
Price float64
}

// 查找 user 时预加载相关 Order
db.Preload("Orders").Find(&users)
// SELECT * FROM users;
// SELECT * FROM orders WHERE user_id IN (1,2,3,4);

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

Joins 预加载

Preload 在一个单独查询中加载关联数据。而 Join Preload 会使用 inner join 加载关联数据,例如:

db.Joins("Company").Joins("Manager").Joins("Account").First(&user, 1)
db.Joins("Company").Joins("Manager").Joins("Account").First(&user, "users.name = ?", "jinzhu")
db.Joins("Company").Joins("Manager").Joins("Account").Find(&users, "users.id IN ?", []int{1,2,3,4,5})

带条件的 Join

db.Joins("Company", DB.Where(&Company{Alive: true})).Find(&users)
// SELECT `users`.`id`,`users`.`name`,`users`.`age`,`Company`.`id` AS `Company__id`,`Company`.`name` AS `Company__name` FROM `users` LEFT JOIN `companies` AS `Company` ON `users`.`company_id` = `Company`.`id` AND `Company`.`alive` = true;

注意 Join Preload 适用于一对一的关系,例如: has one, belongs to

预加载全部

与创建、更新时使用 Select 类似,clause.Associations 也可以和 Preload 一起使用,它可以用来 预加载 全部关联,例如:

type User struct {
gorm.Model
Name string
CompanyID uint
Company Company
Role Role
Orders []Order
}

db.Preload(clause.Associations).Find(&users)

clause.Associations 不会预加载嵌套的关联,但你可以使用嵌套预加载 例如:

db.Preload("Orders.OrderItems.Product").Preload(clause.Associations).Find(&users)

带条件的预加载

GORM 允许带条件的 Preload 关联,类似于内联条件

// 带条件的预加载 Order
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');

自定义预加载 SQL

您可以通过 func(db *gorm.DB) *gorm.DB 实现自定义预加载 SQL,例如:

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;

嵌套预加载

GORM 支持嵌套预加载,例如:

db.Preload("Orders.OrderItems.Product").Preload("CreditCard").Find(&users)

// 自定义预加载 `Orders` 的条件
// 这样,GORM 就不会加载不匹配的 order 记录
db.Preload("Orders", "state = ?", "paid").Preload("Orders.OrderItems").Find(&users)

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