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 left 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})
Join with conditions

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;
Join nested model

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

Preload All
clause.Associations can work 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
Orders []Order
}

db.Preload(clause.Associations).Find(&users)
clause.Associations won’t preload nested associations, but you can use it with Nested Preloading together, e.g:

db.Preload("Orders.OrderItems.Product").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)
Embedded Preloading
Embedded Preloading is used for Embedded Struct, especially the
same struct. The syntax for Embedded Preloading is similar to Nested Preloading, they are divided by dot.

For example:

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)
We can omit embedded part when there is no ambiguity.

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
NOTE Embedded Preload only works with belongs to relation.
Values of other relations are the same in database, we can’t distinguish them.