Preloading (Eager Loading)

Preload

GORM allows eager loading relations in other SQL with Preload, for example:

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

Joins Preloading

Preload loads the association data in a separate query, Join Preload will loads association data using inner join, for example:

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})

NOTE Join Preload works with one-to-one relation, e.g: has one, belongs to

Preload All

clause.Associations can works with Preload similar like Select when creating/updating, you can use it to Preload all associations, for example:

type User struct {
  gorm.Model
  Name       string
  CompanyID  uint
  Company    Company
  Role       Role
}

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

Preload with conditions

GORM allows Preload associations with conditions, it works similar to Inline Conditions

// 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');

Custom Preloading SQL

You are able to custom preloading SQL by passing in func(db *gorm.DB) *gorm.DB, for example:

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;

Nested Preloading

GORM supports nested preloading, for example:

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)