How do I express a “has many through” relationship in Entity Framework 5?

Question

I am attempting to use Entity Framework 5 to query an existing MySQL database. I used code-first to create a code-based model that maps to an existing database following this tutorial on MSDN.

I have two tables: users and buddies. A User has an id, a name and an email. A Buddy has a user_id and a buddy_id. A User has many Buddies (which are also Users). The buddy_id column is a foreign key back into the Users table. So each User has many Users through Buddies.

public class User
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string Email { get; set; }
    public IList<User> Buddies { get; set; } 
}

Here is my database access code:

using (var db = new Models.fooContext())
{
    foreach (Models.user user in db.users)
    {
        var u = new User
        {
            Id      = user.id,
            Name    = user.name,
            Email   = user.email,
            Buddies = new List<User>()
        };

        // this is obviously very inefficient but I am just
        // trying to get anything to work
        foreach (Models.buddy buddy in db.buddies) // this line throws an exception
        {
            if (buddy.user_id == user.id)
            {
                var b = new User();
                // take buddy.user_id and find this user in the user table
                u.Buddies.Add(b);
            }
        }

        // serialize User and write it to a file here
    }
}

This code throws the following exception on the line indicated above:

System.Data.EntityCommandExecutionException: an error occurred while executing the command definition. See the inner exception for details."

The inner exception is a MySql.Data.MySqlClient.MySqlException with the message

There is already an open DataReader associated with this Connection which must be closed first.

My questions:

1. How do I create a relationship that tells EF that each User has many Buddies?
2. Once EF understands that a User has many Buddies, how do I use the buddy_id to find the User record for that Buddy?

Answer 1

You have one data type that you are calling separate names. That is a little confusing. However To get this to work with code first you just need the following fluent configuration in your Custom DbContext class:

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
    modelBuilder.Entity<user>().
      HasMany(c => c.Buddies).
      WithMany().
      Map(
       m =>
       {
          m.MapLeftKey("user_id");
          m.MapRightKey("buddy_id");
          m.ToTable("buddies");
       });
}

This assuming your user class looks like this:

[Table("user")]
public class user
{
    public int id { get; set; }
    public string name { get; set; }
    public string email { get; set; }
    public virtual List<user> Buddies { get; set; }
}

If you use the above method every user object you have will have a navigation property on it called Buddies. When querying for users you will want to eager load buddy users, do:

context.users.Include("Buddies")

Further, To address your issue with multiple readers. It is because you have not enumerated the query (db.users) from your first loop. To address that you can enumerate the query like so:

var users = context.users.Include("Buddies").ToList();
foreach(var user in users)....

And if you use the configuration above, you don't need to try and match id's you simply acquire the list (null field if new) of buddies from the user using the buddies navigation property (virtual,lazy loaded), do:

user.Buddies

As you can see you don't really need a 'Model.buddy' (since it only contains an id mapping) Entity framework will take care of the linking. However if you're not always including the Buddies in your user query you may want the table. Which would be queried with LINQ in the following way:

var userBuddies = db.buddies.Where(buddy=>buddy.user_id == user.id).ToList();
//An enumerated list of user buddies 
//do stuff