How to fix ‘Unable to determine the relationship represented by navigation property’ error in Entity Framework

UPDATE

You have multiple options here:

Option 1 with result 1

City Class becomes:

public partial class City
{
    public City()
    {           
        Connections = new HashSet<Connection>();
    }

    public int Id { get; set; }
    public string Name { get; set; }
    public string Country { get; set; }

    public ICollection<Connection> Connections { get; set; }
}

Connection Class becomes:

public partial class Connection
{
    public Connection()
    {
    }

    public int Id { get; set; }

    public int StartCityId { get; set; }
    public int EndCityId { get; set; }

    public int AantalMinuten { get; set; }
    public double Prijs { get; set; }     
}

Your OnModelCreating becomes:

modelBuilder.Entity<City>().HasMany(city => city.Connections)
                           .WithRequired().HasForeignKey(con => con.EndCityId);

modelBuilder.Entity<City>().HasMany(city => city.Connections)
                           .WithRequired().HasForeignKey(con => con.StartCityId);

OR you can do something like this as well wchich would be option 2 with results 2:

City Class becomes:

public partial class City
{
    public City()
    {           
        Connections = new HashSet<Connection>();
    }

    public int Id { get; set; }
    public string Name { get; set; }
    public string Country { get; set; }

    public ICollection<Connection> Connections { get; set; }
}

Connection Class becomes:

public partial class Connection
{
    public Connection()
    {
    }

    public int Id { get; set; }

    public virtual ICollection<City> Cities { get; set; }

    public int AantalMinuten { get; set; }
    public double Prijs { get; set; }     
}

And you don’t have to do anything in your OnModelCreating.