How to model many-to-many relationships with a relationship entity in EF 4.1 Code First

Question

Popular example: In the issue tracker JIRA, issues can be linked to other issues. The link itself has some data attached, in particular a type.

Example:

Issue A -> depends on -> Issue B
Issue B <- is depended on by <- Issue A

We are introducing the same kind of relationship for an entity in our C# ASP.NET MVC application using EF 4.1 CodeFirst, and I'm wondering how to best model this relationship?

---

Details:

There are some particularities about this situation:

• A link has some data attached, so we can't simply model a many-to-many relationship between issues and issues. We rather have to introduce a new entity Link, which represents a relationship between two issues.
• A link, by definition, links two instances of the same entity, it is a "two-to-many" relationship (a link has two issues, an issue can have many links).
• The link is directed, which means, if Issue A depends on Issue B, then Issue B is depended on by Issue A.

We will certainly have a Link entity that looks like this:

public class Link
{
    public int ID { get; set; }
    public Issue IssueA { get; set; }
    public Issue IssueB { get; set; }
    public LinkType Type { get; set; }
}

The Issue class might look like this:

public class Issue
{
    public int ID { get; set; }
    public virtual ICollection<Link> Links { get; set; }
}

Currently there would be only one link type: dependency. So, the link type would look like this:

public class LinkType
{
    public int ID { get; set; }
    public string ForwardName { get; set; } // depends on
    public string BackwardName { get; set; } // is depended on by
}

Now for the big question:

If I want EF to automatically manage Issue.Links, I have to tell it what Foreign key on the Link table to use. Either I use IssueA, or I use IssueB. I can't use both, can I?

Either I define:

modelBuilder.Entity<Issue>().HasMany(i => i.Links).WithRequired(l => l.IssueA);

or I define:

modelBuilder.Entity<Issue>().HasMany(i => i.Links).WithRequired(l => l.IssueB);

---

Possible approaches - I am curious about your feedback on whether some of them will lead to troubles, cannot be implemented, or whether any of these approaches can be regarded as "best practice":

• Add two Collections to the Issue, ICollection<Link> OutgoingLinks, ICollection<Link> IncomingLinks. This way the collections can be maintained by EF, but from a business logic point of view they don't make much sense.
• Only add one collection and configure EF 4.1 to add incoming and outgoing links to it, if that is possible.

• Only add one collection and implement it on my own:

  ICollection<Link> AllLinks { return _context.Links.Where(l => l.IssueA == this || l.IssueB == this).ToList(); }

  The problem with this approach is that the domain entity executes data access tasks which is bad in terms of seperation of concerns.

• Any other?

Answer 1

Option (1) is the way to go in my opinion, together with a readonly helper perhaps which combines the two collections:

public class Issue
{
    public int ID { get; set; }
    public virtual ICollection<Link> OutgoingLinks { get; set; }
    public virtual ICollection<Link> InComingLinks { get; set; }

    public IEnumerable<Link> Links // not mapped because readonly
    {
        get { return OutgoingLinks.Concat(InComingLinks); }
    }
}

Option (2) isn't possible because you cannot map one navigation property to two different ends/navigation properties.