Method Chaining

Method Chaining

Gorm implements method chaining interface, so you could write code like this:

db, err := gorm.Open("postgres", "user=gorm dbname=gorm sslmode=disable")

// create a new relation
tx := db.Where("name = ?", "jinzhu")

// add more filter
if someCondition {
    tx = tx.Where("age = ?", 20)
} else {
    tx = tx.Where("age = ?", 30)
}

if yetAnotherCondition {
    tx = tx.Where("active = ?", 1)
}

Query won’t be generated until a immediate method, which could be useful in some cases.

Like you could extract a wrapper to handle some common logic

Immediate Methods

Immediate methods are those methods that will generate SQL query and send it to database, usually it is those CRUD methods, like:

Create, First, Find, Take, Save, UpdateXXX, Delete, Scan, Row, Rows…

Here is an immediate methods example based on above chain:

tx.Find(&user)

Generates

SELECT * FROM users where name = 'jinzhu' AND age = 30 AND active = 1;

Scopes

Scope is build based on the method chaining theory.

With it, you could extract some generic logics, to write more reusable libraries.

func AmountGreaterThan1000(db *gorm.DB) *gorm.DB {
  return db.Where("amount > ?", 1000)
}

func PaidWithCreditCard(db *gorm.DB) *gorm.DB {
  return db.Where("pay_mode_sign = ?", "C")
}

func PaidWithCod(db *gorm.DB) *gorm.DB {
  return db.Where("pay_mode_sign = ?", "C")
}

func OrderStatus(status []string) func (db *gorm.DB) *gorm.DB {
  return func (db *gorm.DB) *gorm.DB {
    return db.Scopes(AmountGreaterThan1000).Where("status IN (?)", status)
  }
}

db.Scopes(AmountGreaterThan1000, PaidWithCreditCard).Find(&orders)
// Find all credit card orders and amount greater than 1000

db.Scopes(AmountGreaterThan1000, PaidWithCod).Find(&orders)
// Find all COD orders and amount greater than 1000

db.Scopes(AmountGreaterThan1000, OrderStatus([]string{"paid", "shipped"})).Find(&orders)
// Find all paid, shipped orders that amount greater than 1000

Multiple Immediate Methods

When using multiple immediate methods with GORM, later immediate method will reuse before immediate methods’s query conditions (excluding inline conditions)

db.Where("name LIKE ?", "jinzhu%").Find(&users, "id IN (?)", []int{1, 2, 3}).Count(&count)

Generates

SELECT * FROM users WHERE name LIKE 'jinzhu%' AND id IN (1, 2, 3)

SELECT count(*) FROM users WHERE name LIKE 'jinzhu%'

Thread Safety

All Chain Methods will clone and create a new DB object (shares one connection pool), GORM is safe for concurrent use by multiple goroutines.