# Advanced Usage

- Error Handling
- Transactions
  - A Specific Example
- SQL Builder
  - Run Raw SQL
  - sql.Row & sql.Rows
  - Scan sql.Rows In Iteration
- Generic database interface sql.DB
  - Connection Pool
- Composite Primary Key
- Logger
  - Customize Logger

## Error Handling

After perform any operations, if there are any error happened, GORM will set it to `*DB`'s `Error` field

    if err := db.Where("name = ?", "jinzhu").First(&user).Error; err != nil {
        // error handling...
    }

    // If there are more than one error happened, get all of them with `GetErrors`, it returns `[]error`
    db.First(&user).Limit(10).Find(&users).GetErrors()

    // Check if returns RecordNotFound error
    db.Where("name = ?", "hello world").First(&user).RecordNotFound()

    if db.Model(&user).Related(&credit_card).RecordNotFound() {
        // no credit card found handling
    }

## Transactions

To perform a set of operations within a transaction, the general flow is as below.

    // begin a transaction
    tx := db.Begin()

    // do some database operations in the transaction (use 'tx' from this point, not 'db')
    tx.Create(...)

    // ...

    // rollback the transaction in case of error
    tx.Rollback()

    // Or commit the transaction
    tx.Commit()

### A Specific Example

    func CreateAnimals(db *gorm.DB) err {
      tx := db.Begin()
      // Note the use of tx as the database handle once you are within a transaction

      if err := tx.Create(&Animal{Name: "Giraffe"}).Error; err != nil {
         tx.Rollback()
         return err
      }

      if err := tx.Create(&Animal{Name: "Lion"}).Error; err != nil {
         tx.Rollback()
         return err
      }

      tx.Commit()
      return nil
    }

## SQL Builder

#### Run Raw SQL

Run Raw SQL

    db.Exec("DROP TABLE users;")
    db.Exec("UPDATE orders SET shipped_at=? WHERE id IN (?)", time.Now, []int64{11,22,33})

    // Scan
    type Result struct {
        Name string
        Age  int
    }

    var result Result
    db.Raw("SELECT name, age FROM users WHERE name = ?", 3).Scan(&result)

#### sql.Row & sql.Rows

Get query result as `*sql.Row` or `*sql.Rows`

    row := db.Table("users").Where("name = ?", "jinzhu").Select("name, age").Row() // (*sql.Row)
    row.Scan(&name, &age)

    rows, err := db.Model(&User{}).Where("name = ?", "jinzhu").Select("name, age, email").Rows() // (*sql.Rows, error)
    defer rows.Close()
    for rows.Next() {
        ...
        rows.Scan(&name, &age, &email)
        ...
    }

    // Raw SQL
    rows, err := db.Raw("select name, age, email from users where name = ?", "jinzhu").Rows() // (*sql.Rows, error)
    defer rows.Close()
    for rows.Next() {
        ...
        rows.Scan(&name, &age, &email)
        ...
    }

#### Scan sql.Rows In Iteration

    rows, err := db.Model(&User{}).Where("name = ?", "jinzhu").Select("name, age, email").Rows() // (*sql.Rows, error)
    defer rows.Close()

    for rows.Next() {
      var user User
      db.ScanRows(rows, &user)
      // do something
    }

## Generic database interface sql.DB

Get generic database interface *sql.DB from `*gorm.DB` connection

    // Get generic database object `*sql.DB` to use its functions
    db.DB()

    // Ping
    db.DB().Ping()

#### Connection Pool

    db.DB().SetMaxIdleConns(10)
    db.DB().SetMaxOpenConns(100)

## Composite Primary Key

Set multiple fields as priamry key to enable composite primary key

    type Product struct {
        ID           string `gorm:"primary_key"`
        LanguageCode string `gorm:"primary_key"`
    }

## Logger

Gorm has built-in logger support, by default, it will print happened errors

    // Enable Logger, show detailed log
    db.LogMode(true)

    // Diable Logger, don't show any log
    db.LogMode(false)

    // Debug a single operation, show detailed log for this operation
    db.Debug().Where("name = ?", "jinzhu").First(&User{})

#### Customize Logger

Refer GORM's default logger for how to customize it https://github.com/jinzhu/gorm/blob/master/logger.go

    db.SetLogger(gorm.Logger{revel.TRACE})
    db.SetLogger(log.New(os.Stdout, "\r\n", 0))