上下文

GORM 的上下文支持由 WithContext 方法启用，是一项强大的功能，可以增强 Go 应用程序中数据库操作的灵活性和控制力。它允许在不同的操作模式、超时设置以及甚至集成到钩子/回调和中间件中进行上下文管理。让我们从不同方面深入了解：

单会话模式

Single session mode is appropriate for executing individual operations. It ensures that the specific operation is executed within the context’s scope, allowing for better control and monitoring.

db.WithContext(ctx).Find(&users)

Continuous Session Mode

Continuous session mode is ideal for performing a series of related operations. It maintains the context across these operations, which is particularly useful in scenarios like transactions.

tx := db.WithContext(ctx)
tx.First(&user, 1)
tx.Model(&user).Update("role", "admin")

Context Timeout

Setting a timeout on the context passed to db.WithContext can control the duration of long-running queries. This is crucial for maintaining performance and avoiding resource lock-ups in database interactions.

ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
defer cancel()

db.WithContext(ctx).Find(&users)

Hooks/Callbacks 中的 Context

The context can also be accessed within GORM’s hooks/callbacks. This enables contextual information to be used during these lifecycle events.

func (u *User) BeforeCreate(tx *gorm.DB) (err error) {
  ctx := tx.Statement.Context
  // ... use context
  return
}

Integration with Chi Middleware

GORM’s context support extends to web server middlewares, such as those in the Chi router. This allows setting a context with a timeout for all database operations within the scope of a web request.

func SetDBMiddleware(next http.Handler) http.Handler {
  return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
    timeoutContext, _ := context.WithTimeout(context.Background(), time.Second)
    ctx := context.WithValue(r.Context(), "DB", db.WithContext(timeoutContext))
    next.ServeHTTP(w, r.WithContext(ctx))
  })
}

// Router setup
r := chi.NewRouter()
r.Use(SetDBMiddleware)

// Route handlers
r.Get("/", func(w http.ResponseWriter, r *http.Request) {
  db, ok := r.Context().Value("DB").(*gorm.DB)
  // ... db operations
})

r.Get("/user", func(w http.ResponseWriter, r *http.Request) {
  db, ok := r.Context().Value("DB").(*gorm.DB)
  // ... db operations
})

Note: Setting the Context with WithContext is goroutine-safe. This ensures that database operations are safely managed across multiple goroutines. For more details, refer to the Session documentation in GORM.

Logger Integration

GORM’s logger also accepts Context, which can be used for log tracking and integrating with existing logging infrastructures.

Refer to Logger documentation for more details.