How to map childs/parent class with petapoco? - orm

Question

I have these classes:
class Car {
int ID;
string Name;
}
class Truck : Car {
int MaximumLoad;
}
I have these tables
Car
- ID
- Name
Truck
- CarID
- MaximumLoad
How could I map my classes with my table using PetaPoco ?

Answer 1

If you store Car and Truck in the same table (TPH) you can inherit Truck from Car with minor changes to PetaPOCO source code,
table Vehicle (ID, Discriminator, Name, MaximumLoad)
in PetaPOCO.cs, add
[AttributeUsage(AttributeTargets.Class)]
public class DiscriminatorAttribute : Attribute
{
public int Value { get; private set; }
public DiscriminatorAttribute(object discriminator)
{
Value = (int)discriminator;
}
}
protected bool IsTPHTable<T>()
{
var t = typeof(T);
var a = t.GetCustomAttributes(typeof(DiscriminatorAttribute), true);
return a.Length > 0;
}
protected void AppendDiscriminator<T>(Sql sql)
{
var t = typeof(T);
var a = t.GetCustomAttributes(typeof(DiscriminatorAttribute), true);
sql.Append("Where Discriminator = #0", (a[0] as DiscriminatorAttribute).Value);
}
public IEnumerable<T> Query<T>(Sql sql)
{
if (IsTPHTable<T>())
AppendDiscriminator<T>(sql);
return Query<T>(default(T), sql);
}
// also similar AppendDiscriminator() for Update an Delete.
Then in your Car.cs and Truck.cs, you can write/generate code like this,
public enum VehicleType:int
{
Car,
Truck
}
[TableName("Vehicle")]
[Discriminator(VehicleType.Car)]
public class Car
{
[Column]
public int ID {get; set;}
[Column]
public string Name {get; set;}
public Car()
{
//this.Discriminator = VehicleType.Car;
}
public static new Car SingleOrDefault(object primaryKey) { return repo.SingleOrDefaultById<Car>(primaryKey); }
//...
}
[Discriminator(VehicleType.Truck)]
public class Truck:Car
{
[Column]
public double MaximumLoad {get;set;}
public Truck()
{
//this.Discriminator = VehicleType.Truck;
}
public static new Truck SingleOrDefault(object primaryKey) { return repo.SingleOrDefaultById<Truck>(primaryKey); }
//...
}

Answer 2

To read truck records I would create a Trucks view that combines the two tables. Or have a look at Schotime's muliple result sets :
http://schotime.net/blog/index.php/2011/11/20/petapoco-multiple-result-sets/
For writes I guess you are asking "how can I write to 2 tables in one operation". Off the top of my head I would probably say I would simply perform 2 writes. I think Petapoco will ignore fields that don't map so you may be able to use your truck object for both writes.
Could easily be wrong as I haven't tested this.

Answer 3

I think (have not tested though) that if you do something like this..
repo.Fetch<Truck>(";Select car.*, truck.maximumload from car left join truck on car.id = truck.carid");
or
repo.Fetch<Truck>(";Select car.*, truck.maximumload from car left join truck on car.id = truck.carid where truck.carid = #0", truckid);
I would probably have called my base class vehicle rather than car, buts that's just me.
Hope that helps ?