public class DirCurrency
{
    [Key]
    public int DirCurrencyID { get; set; }
    public bool Del { get; set; }
    // ...
}

public class SysSettings
{
    [Key]
    public int SysSettingsID { get; set; }

    [Display(Name = "Валюта")]
    [Required]
    public int DirCurrencyID { get; set; }
    [ForeignKey("DirCurrencyID")]
    public virtual Dir.DirCurrency dirCurrency { get; set; }

    // ...
}

     //Настройки, нужны для некоторых операций
     SysSettings sysSettings = await db.SysSettingss.FindAsync(1);

     //Поиск
     DirCurrency dirCurrency = await db.DirCurrencies.FindAsync(id);
     if (dirCurrency == null) return NotFound();
     //Удаление
     db.DirCurrencies.Remove(dirCurrency);
     await db.SaveChangesAsync();

     db.DirCurrencies.Remove(dirCurrency);
     await db.SaveChangesAsync();

     //Настройки, нужны для некоторых операций
     SysSettings sysSettings = await db.SysSettingss.FindAsync(1);