EF 4.1 RC: Weird Cascade Delete

问题

I have to admit, the features of EF 4.1 RC Codefirst, DataAnnotations and FluentAPI are still overwhelming to me. Sometimes I really don't know what I am doing ;-) Please see the following POCOs:

public class Country
{
    [Key]
    public Guid ID { get; set; }

    [Required]
    public virtual Currency Currency { get; set; }
}

public class Currency
{
    [Key]
    public Guid ID { get; set; }

    public virtual ICollection<Country> Countries { get; set; }
}

The general idea: Every country needs to have a currency. But a currency does not need to be assigned to a country at all.

If you let EF create the corresponding database, the relationship will be set to CASCADE DELETE by convention. In other words: if you delete a currency, the corresponding countries are deleted as well. But in my case this is not what I want.

I came up with some code in FluentAPI in order to disable CASCADE DELETE:

modelBuilder.Entity<Country>()
            .HasRequired(cou => cou.Currency)
            .WithOptional()
            .WillCascadeOnDelete(false);

I thought this means: Every country requires a currency. And this currency might have zero, one or more countries assigned (optional). And whenever I delete a currency, the corresponding countries (if there are any) will NOT be cascade deleted.

Surprisingly the given approach will still cascade delete a country if I delete the corresponding currency. Can anybody tell me what I miss?

回答1:

Firstly you've specified the currency as a required field on country, so you can't delete a currency. You'll need to remove the [Required].

Secondly, your model builder need the following:

modelBuilder.Entity<Country>()
            .HasRequired(cou => cou.Currency) //note optional, not required
            .WithMany(c=>c.Countries)         //define the relationship
            .WillCascadeOnDelete(false);

Thirdly, you need to explicitly remove the reference to the entity you are deleting from it's children:

 Currency c = context.Currencies.FirstOrDefault();

                c.Countries.Clear(); //these removes the link between child and parent

                context.Currencies.Remove(c);

                context.SaveChanges();

[EDIT] Because I suspect there is something lost in translation find the complete code that demonstrates how no-cascading deletes would work.

public class Country{
  [Key]
  public Guid ID { get; set; }

  public virtual Currency Currency { get; set; }
}

public class Currency{
  [Key]
  public Guid ID { get; set; }

  public virtual ICollection<Country> Countries { get; set; }
}


public class MyContext : DbContext{
  public DbSet<Currency> Currencies { get; set; }
  public DbSet<Country> Countries { get; set; }

  protected override void OnModelCreating(DbModelBuilder modelBuilder){
    modelBuilder.Entity<Country>()
     .HasRequired(country => country.Currency)
     .WithMany(currency => currency.Countries)
     .WillCascadeOnDelete(false);
  }
}

class Program{
  static void Main(string[] args){
    Database.DefaultConnectionFactory = new   SqlCeConnectionFactory("System.Data.SqlServerCe.4.0");

    Database.SetInitializer(new DropCreateDatabaseAlways<MyContext>());

    using (MyContext context1 = new MyContext()){
      Currency c = new Currency{ID = Guid.NewGuid()};

      context1.Currencies.Add(c);

      c.Countries = new List<Country>();

      c.Countries.Add(new Country{ID = Guid.NewGuid()});

      context1.SaveChanges();
   }

   using (MyContext context2 = new MyContext()){
     Currency c = context2.Currencies.FirstOrDefault();

     context2.Currencies.Remove(c);

     //throws exception due to foreign key constraint
     //The primary key value cannot be deleted 
     //because references to this key still exist.   
     //[ Foreign key constraint name = Country_Currency ]

     context2.SaveChanges();
    }          
  }
}

You will get an error on saving, because your deleting something that is a required foreign key.

来源：https://stackoverflow.com/questions/5402964/ef-4-1-rc-weird-cascade-delete