具有多对多关系的数据添加/删除

Question

I'm trying to use Entity Framework 4.0 to add and remove records with a many-to-many relationship. I've scoured the web but am just not finding a meaningful example.

As an example, let's say I have a Drivers and Vehicles table with a junction table VehicleDrivers.

Drivers

• DriverID
• ...

Vehicles

• VehicleID
• ...

VehicleDrivers

• DriverID
• VehicleID

My questions are:

1. How would I write EF statements to add a new vehicle and related driver?
2. How would I write EF statements to delete an existing vehicle and any related drivers?

Thanks for any tips.

Answer 1

Something like this:

How would I write EF statements to add a new vehicle and related driver?

For existing driver:

using (var context = new MyContext(...))
{
    var newVehicle = new Vehicle { ... };
    var existingDriver = context.Drivers.First(d => d.Name == "Jim");
    newVehicle.Drivers.Add(existingDriver);

    context.Vehicles.AddObject(newVehicle);

    context.SaveChanges();
}

For new driver:

using (var context = new MyContext(...))
{
    var newVehicle = new Vehicle { ... };
    var newDriver = new Driver { ... };
    newVehicle.Drivers.Add(newDriver);

    context.Vehicles.AddObject(newVehicle);

    context.SaveChanges();
}

How would I write EF statements to delete an existing vehicle and any related drivers?

using (var context = new MyContext(...))
{
    var existingVehicle = context.Vehicles.First(v => v.Name == "Ford");

    context.Vehicles.DeleteObject(existingVehicle);

    context.SaveChanges();
}

Nothing more to do here because the related entries in the join table with all drivers linked to the deleted vehicle will be deleted as well due to enabled cascading delete in the database.

Edit

If you want to delete the drivers as well (and not only the links) in a many-to-many relationship you need to check first if the driver isn't linked to another vehicle:

using (var context = new MyContext(...))
{
    var existingVehicle = context.Vehicles.Include("Drivers")
        .First(v => v.Name == "Ford");

    foreach(var driver in existingVehicle.Drivers.ToList())
    {
        if (!context.Drivers.Any(d => d.DriverId == driver.DriverId
            && d.Vehicles.Any(v => v.VehicleId != existingVehicle.VehicleId)))
            context.Drivers.DeleteObject(driver);
    }
    context.Vehicles.DeleteObject(existingVehicle);

    context.SaveChanges();
}

Answer 2

What I usually do is use the key field and place it into the object I'm saving. This does not work well when you are modifying several new objects at the same time however.