Update entity on db with a property of list of entities with one-to-many relationship

Question

I will make up some simpler entities to get a better understanding:

public class Room
{
    [Required]
    public int Id { get; set; }

    [Required]
    public int Name { get; set; }

    [Required]
    [Column("House_Id")]
    public int HouseId { get; set; }
    public virtual House House { get; set; }
}


public class House
{
    [Required]
    public int Id{ get; set; }

    public string Name { get; set; }

    public virtual ICollection<Room> Rooms { get; set; }
}

Now, lets say, that i have one House entity with 4 rooms related to it in database. I want to update the House and remove 2 of its rooms. I'm using Asp.Net MVC4 btw.

        var house = HouseRepository.Get(id);
        house.Name = model.Name; // Some string
        house.Name = model.Rooms; // ICollection<Room> from view with 2 rooms in it, cuz user deleted other two in view
        HouseRepository.SaveChanges();
        return house;

What happens, is that i get error saying : The operation failed: The relationship could not be changed because one or more of the foreign-key properties is non-nullable. When a change is made to a relationship, the related foreign-key property is set to a null value. If the foreign-key does not support null values, a new relationship must be defined, the foreign-key property must be assigned another non-null value, or the unrelated object must be deleted.

I need to find a solution how to delete the unrelated Rooms from db. When and where i should do it? I've tried something already, but now i get that error even I'm adding rooms.

Is there any simple way to tell EF that it should delete these rooms by itself?

Answer 1

EF can delete orphans automatically only if you use the identifying relationship. You need to define a composite key for the Room object, that consists of Id and HouseID .

modelbuilder.Entity<Room>()
            .HasKey(o => new { o.ID, o.HouseId });

modelbuilder.Entity<Room>()
            .Property(o => o.ID)
            .HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity);

With this configuration EF knows, that when you remove a room object from the Rooms collection, it's safe to remove the room for the DB as well.

Answer 2

Delete the Rooms you no longer want individually to explicitly remove them from the model - something equivalent to this:

foreach(Room room in house.Rooms.ToList())
{
    if(model.Rooms.FirstOrDefault(x=>x.Id == room.Id) == null)
         dbContext.Rooms.Remove(room); //assumes the room is attached to your context, you may need to do a get
}

Answer 3

Assuming your setup of 1 house that has 4 rooms currently associated to it, this should work:

var db = new TestContext();
var house = db.Houses.Include("Rooms").First();

// Grab rooms 2 & 3 for removal
var roomsToRemove = house.Rooms.OrderBy(r => r.Id).Skip(1).Take(2).ToList();
roomsToRemove.ForEach(r => db.Rooms.Remove(r));

db.SaveChanges();