CourseWork_Bank/Bank/BankDatabaseImplement/Models/CostByPurchase.cs

using System.ComponentModel.DataAnnotations;
using BankDataModels.ProxyModels;

namespace BankDatabaseImplement.Models
{
    public class CostByPurchase : CostByPurchaseModel
    {
        [Required]
        public Cost? Cost { get; private set; }
        [Required]
        public Purchase? Purchase { get; private set; }
    }
}