Many-To-Many Sorting

Question

ASP.NET MVC4, C#.

I have two models A and B. They have a many-to-many relationship with each other.

Suppose A has a collection of B. I want to sort the collection of B on A and persist it in a database.

The correct database design for this is to add an additional property to the bridge table A_B. However, entity framework does not allow this. What is the best solution to allow sorting in entity framework for many-to-many relationships?

Thanks.

Answer 1

You must model this with two one-to-many relationships instead of a single many-to-many. It means that you have to expose the bridge table as an entity in your model and this entity would have a SortOrder property:

public class A_B
{
    [Key, ForeignKey("A"), Column(Order = 1)]
    public int AId { get; set; }
    [Key, ForeignKey("B"), Column(Order = 2)]
    public int BId { get; set; }

    public A A { get; set; }
    public B B { get; set; }

    public int SortOrder { get; set; }
}

The entities A and B get collections not directly to each other but to this intermediate entity:

public class A
{
    public int AId { get; set; }
    public ICollection<A_B> A_Bs { get; set; }
}

public class B
{
    public int BId { get; set; }
    public ICollection<A_B> A_Bs { get; set; }
}

To save the relationship "in sorted order" you could then write for example:

var b1 = context.Bs.Find(1);
var b2 = context.Bs.Find(2);

var a = new A
{
    A_Bs = new List<A_B>
    {
        new A_B { B = b1, SortOrder = 1 },
        new A_B { B = b2, SortOrder = 2 }
    }
};

context.As.Add(a);
context.SaveChanges();

Now, when you load the A entity you can load the collection in that sort order:

var a = context.As.Find(1);
a.A_Bs = context.Entry(a).Collection(x => x.A_Bs).Query()
    .Include(y => y.B)
    .OrderBy(y => y.SortOrder)
    .ToList();

Here are more details how to work with such a model that has additional data in the bridge table.