Setting Interface as an attribute having many implementations using GORM

Question

I have a Postgres database which stores JSON as one of its fields. The structure of the table is:

type Table struct {
    ID int
    VehicleType string
    Vehicle     Vehicle `gorm:"type:jsonb"`
//  ......  
}

Now, Vehicle is an interface

type Vehicle interface {
     GetEngineModel() string
}

It has many implementation, I will share one of them - Car

type Car struct {
     CarEngineModel  string //the attributes will be different for different 
                            //implementation
}
func (car Car) GetEngineModel() string {
    return car.CarEngineModel
}

In order to parse the attributes in a specific struct i.e. Car, Bike, .., I can implement Scan and Value interface of all the implementations something like this :-

func (car *Car) Scan(value interface{}) error {
   //Want to use this implementation of Scan based on VehicleType
   //attribute of struct table
   b, ok := value.([]byte)
   if !ok {
      return errors.New("type assertion to []byte failed")
   }
   *car = json.Unmarshal(b, &car)
}

Is there a way to tell which implementation of Scan to use based on other table columns or an alternate way to do the same using GORM? I only want one table(genetic json type) so don't want to use different tables for different implementations using polymorphic association.

Answer 1

You can add a separate field that will hold the raw JSON data and then implement gorm specific hooks to marshal/unmarshal that data.

type Table struct {
    ID          int
    VehicleType string
    Vehicle     Vehicle `gorm:"-"`
    // ...

    VehicleRaw []byte `gorm:"column:vehicle"`
}

func (t *Table) BeforeSave(tx *gorm.DB) (err error) {
    raw, err := json.Marshal(t.Vehicle)
    if err != nil {
        return err
    }
    
    t.VehicleRaw = raw
    return nil
}

func (t *Table) AfterFind(tx *gorm.DB) (err error) {
    switch t.VehicleType {
    case "CAR":
        t.Vehicle = &Car{}
    case "BIKE":
        t.Vehicle = &Bike{}
    }
    return json.Unmarshal(t.VehicleRaw, t.Vehicle)
}