Callbacks

GORM leverages Callbacks to power its core functionalities. These callbacks provide hooks for various database operations like Create, Query, Update, Delete, Row, and Raw, allowing for extensive customization of GORM’s behavior.

Callbacks are registered at the global *gorm.DB level, not on a session basis. This means if you need different callback behaviors, you should initialize a separate *gorm.DB instance.

Registering a Callback

You can register a callback for specific operations. For example, to add a custom image cropping functionality:

func cropImage(db *gorm.DB) {
  if db.Statement.Schema != nil {
    // crop image fields and upload them to CDN, dummy code
    for _, field := range db.Statement.Schema.Fields {
      switch db.Statement.ReflectValue.Kind() {
      case reflect.Slice, reflect.Array:
        for i := 0; i < db.Statement.ReflectValue.Len(); i++ {
          // Get value from field
          if fieldValue, isZero := field.ValueOf(db.Statement.Context, db.Statement.ReflectValue.Index(i)); !isZero {
            if crop, ok := fieldValue.(CropInterface); ok {
              crop.Crop()
            }
          }
        }
      case reflect.Struct:
        // Get value from field
        if fieldValue, isZero := field.ValueOf(db.Statement.Context, db.Statement.ReflectValue); !isZero {
          if crop, ok := fieldValue.(CropInterface); ok {
            crop.Crop()
          }
        }
        // Set value to field
        err := field.Set(db.Statement.Context, db.Statement.ReflectValue, "newValue")
      }
    }
    // All fields for current model
    db.Statement.Schema.Fields
    // All primary key fields for current model
    db.Statement.Schema.PrimaryFields
    // Prioritized primary key field: field with DB name `id` or the first defined primary key
    db.Statement.Schema.PrioritizedPrimaryField
    // All relationships for current model
    db.Statement.Schema.Relationships
    // Find field with field name or db name
    field := db.Statement.Schema.LookUpField("Name")
    // processing
  }
}
// Register the callback for the Create operation
db.Callback().Create().Register("crop_image", cropImage)

Deleting a Callback

If a callback is no longer needed, it can be removed:

// Remove the 'gorm:create' callback from Create operations
db.Callback().Create().Remove("gorm:create")

Replacing a Callback

Callbacks with the same name can be replaced with a new function:

// Replace the 'gorm:create' callback with a new function
db.Callback().Create().Replace("gorm:create", newCreateFunction)

Ordering Callbacks

Callbacks can be registered with specific orders to ensure they execute at the right time in the operation lifecycle.

// Register to execute before the 'gorm:create' callback
db.Callback().Create().Before("gorm:create").Register("update_created_at", updateCreated)
// Register to execute after the 'gorm:create' callback
db.Callback().Create().After("gorm:create").Register("update_created_at", updateCreated)
// Register to execute after the 'gorm:query' callback
db.Callback().Query().After("gorm:query").Register("my_plugin:after_query", afterQuery)
// Register to execute after the 'gorm:delete' callback
db.Callback().Delete().After("gorm:delete").Register("my_plugin:after_delete", afterDelete)
// Register to execute before the 'gorm:update' callback
db.Callback().Update().Before("gorm:update").Register("my_plugin:before_update", beforeUpdate)
// Register to execute before 'gorm:create' and after 'gorm:before_create'
db.Callback().Create().Before("gorm:create").After("gorm:before_create").Register("my_plugin:before_create", beforeCreate)
// Register to execute before any other callbacks
db.Callback().Create().Before("*").Register("update_created_at", updateCreated)
// Register to execute after any other callbacks
db.Callback().Create().After("*").Register("update_created_at", updateCreated)

Predefined Callbacks

GORM comes with a set of predefined callbacks that drive its standard features. It’s recommended to review these defined callbacks before creating custom plugins or additional callback functions.

Plugins

GORM’s plugin system allows for easy extensibility and customization of its core functionalities, enhancing your application’s capabilities while maintaining a modular architecture.

The Plugin Interface

To create a plugin for GORM, you need to define a struct that implements the Plugin interface:

type Plugin interface {
  Name() string
  Initialize(*gorm.DB) error
}

• Name Method: Returns a unique string identifier for the plugin.
• Initialize Method: Contains the logic to set up the plugin. This method is called when the plugin is registered with GORM for the first time.

Registering a Plugin

Once your plugin conforms to the Plugin interface, you can register it with a GORM instance:

// Example of registering a plugin
db.Use(MyCustomPlugin{})

Accessing Registered Plugins

After a plugin is registered, it is stored in GORM’s configuration. You can access registered plugins via the Plugins map:

// Access a registered plugin by its name
plugin := db.Config.Plugins[pluginName]

Practical Example

An example of a GORM plugin is the Prometheus plugin, which integrates Prometheus monitoring with GORM:

// Registering the Prometheus plugin
db.Use(prometheus.New(prometheus.Config{
  // Configuration options here
}))

Prometheus plugin documentation provides detailed information on its implementation and usage.