Entity framework Entities in 'xxContext.x' participate in the 'x_y' relationship error - entity-framework-4

Question

Before me, one developer used Entity Framework code first. I am not good at EF code first, so when I try to insert data, my code gives this error:
Entities in 'TourismContext.HotelOrders' participate in the 'HotelOrder_Order' relationship. 0 related 'HotelOrder_Order_Target' were found. 1 'HotelOrder_Order_Target' is expected.
This is my insert code:
var hotelOrdersInsert = new Data.Entities.HotelOrder
{
OrderId = odr.ID // this gives 7
HotelID = 13,
StartAt = DateTime.Now, // arrivalDate,
EndAt = DateTime.Now, // departureDate,
PaymentTypeID = 1,
PaymentStatusID = 1,
PaymentIdentifier = "a",
TotalRate = Convert.ToDecimal(total),
CurrencyID = 1
};
db.HotelOrders.Add(hotelOrdersInsert);
db.SaveChanges();
And this is my HotelOrder class:
public class HotelOrder
{
[Key, DatabaseGenerated(DatabaseGeneratedOption.Identity)]
public int ID { get; set; }
[Required]
public int HotelID { get; set; }
public int OrderId { get; set; }
// other properties
public virtual Order Order { get; set; }
public virtual Hotel Hotel { get; set; }
}
This is my Order class:
public class Order
{
[Key, DatabaseGenerated(DatabaseGeneratedOption.Identity)]
public int ID { get; set; }
public virtual HotelOrder HotelOrder { get; set; }
}
Where can I find the relationship between the order and hotel order models?

Answer 1

You must write this relation as follows:
public class HotelOrder
{
[Key, DatabaseGenerated(DatabaseGeneratedOption.Identity)]
public int ID { get; set; }
[Required]
public int HotelID { get; set; }
[ForeignKey("Order")]
public int OrderId { get; set; }
public Order Order { get; set; }
[ForeignKey("Hotel")]
public int HotelId { get; set; }
public Hotel Hotel { get; set; }
}
public class Order
{
[Key, DatabaseGenerated(DatabaseGeneratedOption.Identity)]
public int ID { get; set; }
public List<HotelOrder> HotelOrders { get; set; }
}

Answer 2

The problem is the relationship between HotelOrder and Order. It's most likely that you have a 1 to 1 relation. Meaning, Every HotelOrder must have an Order.
The relation is probably defined in the so called fluent API. Check the OnModelCreating(DbModelBuilder) function override of your class derived from DbContext. It probably states something like this:
modelBuilder.Entity<HotelOrder>().HasRequired(hotel_order => hotel_order.Order).WithRequiredPrincipal();
This tells EntityFramework that every HotelOrder must have an Order.
Alternatively, the relationship could be defined as DataAnnotations. Like Elvin Mammadov explained in his answer.
At this point the solution becomes obvious. You need to add an instance of Order to your HotelOrder instance before adding it to db.HotelOrders.