预加载（渴望加载）

预加载

GORM 允许使用 Preload 在其他 SQL 中进行渴望加载关系，例如

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

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

// Preload Orders when find users
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

连接预加载

Preload 在单独的查询中加载关联数据，Join Preload 将使用左连接加载关联数据，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})

带条件的连接

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;

连接嵌套模型

db.Joins("Manager").Joins("Manager.Company").Find(&users)
// SELECT "users"."id","users"."created_at","users"."updated_at","users"."deleted_at","users"."name","users"."age","users"."birthday","users"."company_id","users"."manager_id","users"."active","Manager"."id" AS "Manager__id","Manager"."created_at" AS "Manager__created_at","Manager"."updated_at" AS "Manager__updated_at","Manager"."deleted_at" AS "Manager__deleted_at","Manager"."name" AS "Manager__name","Manager"."age" AS "Manager__age","Manager"."birthday" AS "Manager__birthday","Manager"."company_id" AS "Manager__company_id","Manager"."manager_id" AS "Manager__manager_id","Manager"."active" AS "Manager__active","Manager__Company"."id" AS "Manager__Company__id","Manager__Company"."name" AS "Manager__Company__name" FROM "users" LEFT JOIN "users" "Manager" ON "users"."manager_id" = "Manager"."id" AND "Manager"."deleted_at" IS NULL LEFT JOIN "companies" "Manager__Company" ON "Manager"."company_id" = "Manager__Company"."id" WHERE "users"."deleted_at" IS NULL

注意 Join Preload 适用于一对一关系，例如：has one、belongs to

预加载所有

clause.Associations 可以与 Preload 一起使用，类似于创建/更新时的 Select，您可以使用它来 Preload 所有关联，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 不会预加载嵌套关联，但您可以将其与嵌套预加载一起使用，例如

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

带条件的预加载

GORM 允许使用条件预加载关联，它的工作方式类似于内联条件

// Preload Orders with conditions
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

您可以通过传入 func(db *gorm.DB) *gorm.DB 来自定义预加载 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)

// Customize Preload conditions for `Orders`
// And GORM won't preload unmatched order's OrderItems then
db.Preload("Orders", "state = ?", "paid").Preload("Orders.OrderItems").Find(&users)

嵌入式预加载

嵌入式预加载用于嵌入式结构体，尤其是
相同的结构体。嵌入式预加载的语法类似于嵌套预加载，它们用点分隔。

例如

type Address struct {
  CountryID int
  Country   Country
}

type Org struct {
  PostalAddress   Address `gorm:"embedded;embeddedPrefix:postal_address_"`
  VisitingAddress Address `gorm:"embedded;embeddedPrefix:visiting_address_"`
  Address         struct {
    ID int
    Address
  }
}

// Only preload Org.Address and Org.Address.Country
db.Preload("Address.Country")  // "Address" is has_one, "Country" is belongs_to (nested association)

// Only preload Org.VisitingAddress
db.Preload("PostalAddress.Country") // "PostalAddress.Country" is belongs_to (embedded association)

// Only preload Org.NestedAddress
db.Preload("NestedAddress.Address.Country") // "NestedAddress.Address.Country" is belongs_to (embedded association)

// All preloaded include "Address" but exclude "Address.Country", because it won't preload nested associations.
db.Preload(clause.Associations)

当没有歧义时，我们可以省略嵌入部分。

type Address struct {
  CountryID int
  Country   Country
}

type Org struct {
  Address Address `gorm:"embedded"`
}

db.Preload("Address.Country")
db.Preload("Country") // omit "Address" because there is no ambiguity

注意 Embedded Preload 仅适用于 belongs to 关系。
其他关系的值在数据库中是相同的，我们无法区分它们。

预加载