Write Plugins

Callbacks

GORM itself is powered by Callbacks, it has callbacks for Create, Query, Update, Delete, Row, Raw, you could fully customize GORM with them as you want

Callbacks are registered into the global *gorm.DB, not the session-level, if you require *gorm.DB with different callbacks, you need to initialize another *gorm.DB

Register Callback

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
  }
}

db.Callback().Create().Register("crop_image", cropImage)
// register a callback for Create process

Delete Callback

Delete a callback from callbacks

db.Callback().Create().Remove("gorm:create")
// delete callback `gorm:create` from Create callbacks

Replace Callback

Replace a callback having the same name with the new one

db.Callback().Create().Replace("gorm:create", newCreateFunction)
// replace callback `gorm:create` with new function `newCreateFunction` for Create process

Register Callback with orders

// before gorm:create
db.Callback().Create().Before("gorm:create").Register("update_created_at", updateCreated)

// after gorm:create
db.Callback().Create().After("gorm:create").Register("update_created_at", updateCreated)

// after gorm:query
db.Callback().Query().After("gorm:query").Register("my_plugin:after_query", afterQuery)

// after gorm:delete
db.Callback().Delete().After("gorm:delete").Register("my_plugin:after_delete", afterDelete)

// before gorm:update
db.Callback().Update().Before("gorm:update").Register("my_plugin:before_update", beforeUpdate)

// before gorm:create and after gorm:before_create
db.Callback().Create().Before("gorm:create").After("gorm:before_create").Register("my_plugin:before_create", beforeCreate)

// before any other callbacks
db.Callback().Create().Before("*").Register("update_created_at", updateCreated)

// after any other callbacks
db.Callback().Create().After("*").Register("update_created_at", updateCreated)

Defined Callbacks

GORM has defined some callbacks to power current GORM features, check them out before starting your plugins

Plugin

GORM provides a Use method to register plugins, the plugin needs to implement the Plugin interface

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

The Initialize method will be invoked when registering the plugin into GORM first time, and GORM will save the registered plugins, access them like:

db.Config.Plugins[pluginName]

Checkout Prometheus as example