Model.cs

public class Customer
{
    public int CustomerId { get; set; }
    public string Name { get; set; }
    public ICollection<Order> Orders { get; set; } = new HashSet<Order>();
}

public class Order
{
    public int OrderId { get; set; }
    public DateTime DateCreated { get; set; }
    public int CustomerId { get; set; }
    public Customer Customer { get; set; }
    public ICollection<OrderItem> OrderItems { get; set; } = new HashSet<OrderItem>();
}

public class OrderItem
{
    public int OrderItemId { get; set; }
    public string Item { get; set; }
    public decimal Price { get; set; }
}