Models: Add IDelivery for DeliveryParts

Elwig/Models/Entities/DeliveryPart.cs

@@ -6,7 +6,7 @@ using System.Linq;
 
 namespace Elwig.Models.Entities {
     [Table("delivery_part"), PrimaryKey("Year", "DId", "DPNr")]
-    public class DeliveryPart {
+    public class DeliveryPart : IDelivery {
         [Column("year")]
         public int Year { get; set; }