实体框架：防止在删除实体后将外键设置为null

Question

我需要实现软删除行为。为此，我如下重写DbContext的SaveChanges（）方法：

public override int SaveChanges()
{
    var deletableEntries = ChangeTracker.Entries<IDeletable>().Where(e => e.State == EntityState.Deleted);
    foreach (var entry in deletableEntries)
    {
        entry.State = EntityState.Modified;
        entry.Entity.Deleted = true;
    }

    return base.SaveChanges();
}

但是，当从DbSet中删除实体时，其外键将重置为null：

var context = new MyContext();
var entity = context.Citizens.FirstOrDefault(e => e.CityCode == "001");
if (entity != null)
{
    context.Citizens.Remove(entity); // After that, entity.CityCode becomes Null
    context.SaveChanges();
}

为什么会发生这种情况以及如何预防呢？

型号：

public class City
{
    [DatabaseGenerated(DatabaseGeneratedOption.None)]
    [Key]
    public string Code { get; set; }
    public string Name { get; set; }
    public virtual ICollection<Citizen> Citizens { get; set; }
}

public class Citizen: IDeletable
{
    public int Id { get; set; }
    [Required]
    public string CityCode { get; set; }
    public virtual City City { get; set; }
    public string Name { get; set; }
    public bool Deleted { get; set; }
}

模型配置：

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
    base.OnModelCreating(modelBuilder);
    modelBuilder.Entity<Citizen>()
        .HasRequired(e => e.City)
        .WithMany(e => e.Citizens)
        .HasForeignKey(e => e.CityCode)
        .WillCascadeOnDelete(false);
}

实体框架版本：6.1.3