type (
	HookFuncSelect func(ctx context.Context, in *HookSelectInput) (result Result, err error)
	HookFuncInsert func(ctx context.Context, in *HookInsertInput) (result sql.Result, err error)
	HookFuncUpdate func(ctx context.Context, in *HookUpdateInput) (result sql.Result, err error)
	HookFuncDelete func(ctx context.Context, in *HookDeleteInput) (result sql.Result, err error)
)

// HookHandler manages all supported hook functions for Model.
type HookHandler struct {
	Select HookFuncSelect
	Insert HookFuncInsert
	Update HookFuncUpdate
	Delete HookFuncDelete
}

// Hook sets the hook functions for current model.
func (m *Model) Hook(hook HookHandler) *Model

// Hook function definition.
var hook = gdb.HookHandler{
	Select: func(ctx context.Context, in *gdb.HookSelectInput) (result gdb.Result, err error) {
		result, err = in.Next(ctx)
		if err != nil {
			return
		}
		for i, record := range result {
			if !record["birth_day"].IsEmpty() {
				age := gtime.Now().Sub(record["birth_day"].GTime()).Hours() / 24 / 365
				record["age"] = gvar.New(age)
			}
			result[i] = record
		}
		return
	},
}
// It registers hook function, creates and returns a new `model`.
model := g.Model("user").Hook(hook)

// The hook function takes effect on each ORM operation when using the `model`.
all, err := model.Where("status", "online").OrderAsc(`id`).All()