MVC EF Code First one to one relationship error

Question

I want to have a list of stands (at a trade show) and a list of exhibitors. The list of stands is separate to the list of exhibitors - however, once registered, I want the exhibitor to be able to book a stand. When they select/book a stand - I would like to then be able to have a list the stands in my view, and also show the associated exhibitor who has booked it. Likewise, I would like to list in another view, the exhibitors, and also which stand they have booked. So I'm trying to setup a one to one relationship (using EF CodeFirst). However, when trying to add a controller for either the Stand or the Exhibitor, I get the following error: My models are: ``` public class Stand { public int StandID { get; set; } public string Description { get; set; } public bool Booked { get; set; } public int ExhibitorID { get; set; } public virtual Exhibitor Exhibitor { get; set; } } public class Exhibitor { public int ExhibitorID { get; set; } public string Company { get; set; } public int StandID…

Accepted Answer

EF doesn't know which entity is the principal (parent) and which is the dependent (child). You need to declare a foreign key on the item that entity that should come first. You can do this with an annotation or a fluent mapping.

Annotation

Add the following namespace:

```
using System.ComponentModel.DataAnnotations.Schema;
```

Annotate your `Stand` class with the following annotation:

```
public class Stand
{
    [ForeignKey("Exhibitor")]
    public int StandID { get; set; }
    public string Description { get; set; }
    public bool Booked { get; set; }
    public int ExhibitorID { get; set; }
    public virtual Exhibitor Exhibitor { get; set; }

}
```

Fluent Mapping

Override your `OnModelCreating` method in your `DbContext` class to include:

```
modelBuilder.Entity<Stand>()
    .HasOptional(s => s.Exhibitor)
    .WithRequired(e => e.Stand);
```

MVC EF Code First one to one relationship error

Solution

Problem