问如何使用“实体框架核心”(又名EF7)实现“软删除”？

EN

public class Attachment : AttachmentBase
{}

public abstract class AttachmentBase
{
    public const string StatePropertyName = "state";

    public Guid Id { get; set; }
}

public enum AttachmentState
{
    Available,
    Deleted
}

public class AttachmentsDbContext : DbContext
{
    public AttachmentsDbContext(DbContextOptions options)
        : base(options)
    {
    }

    public DbSet<Attachment> Attachments { get; set; }

    public override Task<int> SaveChangesAsync(CancellationToken cancellationToken = default(CancellationToken))
    {
        IEnumerable<EntityEntry<Attachment>> softDeletedAttachments = ChangeTracker.Entries<Attachment>().Where(entry => entry.State == EntityState.Deleted);

        foreach (EntityEntry<Attachment> softDeletedAttachment in softDeletedAttachments)
        {
            softDeletedAttachment.State = EntityState.Modified;
            softDeletedAttachment.Property<int>(AttachmentBase.StatePropertyName).CurrentValue = (int)AttachmentState.Deleted;
        }
        return base.SaveChangesAsync(cancellationToken);
    }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<AttachmentBase>()
            .HasDiscriminator<int>(AttachmentBase.StatePropertyName)
            .HasValue<Attachment>((int)AttachmentState.Available);

        modelBuilder.Entity<AttachmentBase>().Property<int>(AttachmentBase.StatePropertyName).Metadata.IsReadOnlyAfterSave = false;

        modelBuilder.Entity<Attachment>()
            .ToTable("available_attachment");

        modelBuilder.Entity<AttachmentBase>()
            .ToTable("attachment");

        base.OnModelCreating(modelBuilder);
    }
}