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;