EF Code First foreign key with multiple keys

Question

I have an event table that can have multiple event dates for it. I've done some research into this error, but it looks like I have everything set up correctly, however I'm new to the code first approach, and I think my confusion comes in with overriding the OnModelCreating

One or more validation errors were detected during model generation:

Event_EventDates_Source_Event_EventDates_Target: : The number of properties in the Dependent and Principal Roles in a relationship constraint must be identical.



[Table("EventDates")]
public class EventDate
{
    [Key, Column(Order=1)]
    public int EventDateId { get; set; }
    [Key, Column(Order = 2)]
    public DateTime EventDateStart { get; set; }
    public DateTime? EventEnd { get; set; }
    public string TicketPurchaseUrl { get; set; }
}

[Table("Events")]
public class Event
{
    [Key, DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int EventId { get; set; }
    ...
    public int EventDateId { get; set; }
    public DateTime EventDateStart { get; set; }
    [ForeignKey("EventDateId, EventDateStart")]
    public virtual ICollection<EventDate> EventDates { get; set; }
}

Answer 1

Your model is a bit weird for the business logic you describe. If an Event can have many EventDates you should not have a foreign key in Event that refers to EventDate, but the other way around: A foreign key in EventDate that refers to Event and possibly also a navigation property Event in EventDate:

[Table("EventDates")]
public class EventDate
{
    public int EventDateId { get; set; }

    public int EventId { get; set; }
    [ForeignKey("EventId")]
    public Event Event { get; set; }

    public DateTime EventDateStart { get; set; }
    public DateTime? EventEnd { get; set; }
    public string TicketPurchaseUrl { get; set; }
}

[Table("Events")]
public class Event
{
    public int EventId { get; set; }

    public virtual ICollection<EventDate> EventDates { get; set; }
}

By convention EventDate.EventDateId and Event.EventId will be the keys and they will be autogenerated. I suggest that you save yourself the trouble of having a DateTime as part of a composite key in EventDate. If you do not really need such a composite key keep it simple with just an int identity as key.