Preloading (Eager loading)

Preload

// the struct User and Order for below code
type User struct {
  gorm.Model
  Username string
  Orders []Order
}
type Order struct {
  gorm.Model
  UserID uint
  Price float64
}
// the Preload function's param should be the main struct's field name
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

Auto Preloading

Always auto preload associations

type User struct {
  gorm.Model
  Name       string
  CompanyID  uint
  Company    Company `gorm:"PRELOAD:false"` // not preloaded
  Role       Role                           // preloaded
}

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

Nested Preloading

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

Custom Preloading SQL

You could 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;