Ttykuu

I've got this all working, but it seems to be quite long-winded, and I thought I'd post here and see if I'm doing it wrong...

I have a M:M relationship between an Installer and a MasterInstance. The classes (code-first) look like:

public class MasterInstance

 .. rest of fields here ..
 public virtual ICollection<Installer> PermittedInstallers get; set; 


public class Installer

 .. rest of fields here ..
 public virtual ICollection<MasterInstance> PermittedMasterInstances get; set; 

I want to be able to edit these in my Installer view, using a multi-list box. So, I create a ViewModel for what I need:

public class InstallerViewModel

 public Installer Installer get; set; 
 public List<MasterInstance> PermittedMasterInstances get; set; 
 public int[] SelectedMasterInstances get; set; 

And then I place this in my view like so so that I can select the instances for my installer:

<div class="editor-field">
 @Html.ListBoxFor(model => model.SelectedMasterInstances,(Model.PermittedMasterInstances).Select(option => new SelectListItem 
 Text = option.Name, 
 Value = option.MasterInstanceId.ToString()
))
</div>

In the controller, for the edit action, I need to set this view model up:

var installer = context.Installers.Include(i => i.PermittedMasterInstances).Single(x => x.InstallerId == installerId);
 InstallerViewModel model = new InstallerViewModel
 
 Installer = installer,
 PermittedMasterInstances = context.MasterInstances.ToList(),
 SelectedMasterInstances = installer.PermittedMasterInstances.Select(i => i.MasterInstanceId).ToArray()
 ;
return View(model);

Finally, on the post of the edit, I need to delete any relationships that are no longer there and add the new ones:

// Grab the model from the viewmodel and attach to the context
var installer = installerModel.Installer;
context.Installers.Attach(installer);

// Load the related records (dont know why Lazy Loading wouldn't kick in here)
context.Entry(installer).Collection(i => i.PermittedMasterInstances).Load();
// Iterate and delete existing relationships
var instancesToDelete = installer.PermittedMasterInstances.Where(mi => !installerModel.SelectedMasterInstances.Contains(i.MasterInstanceId)).ToList();
instancesToDelete.ForEach(mi => installer.PermittedMasterInstances.Remove(mi));

// Now loop through an int[] and add those new relations, WITHOUT the pain of fetching them from the DB
foreach (var permittedMasterInstanceId in installerModel.SelectedMasterInstances)

 if (!installer.PermittedMasterInstances.Any(pmi => pmi.MasterInstanceId == permittedMasterInstanceId))
 
 var masterInstance = new MasterInstance MasterInstanceId = permittedMasterInstanceId;
 context.MasterInstances.Attach(masterInstance);
 installer.PermittedMasterInstances.Add(masterInstance);
 


// We're done - save and finish.
context.Entry<Installer>(installer).State = EntityState.Modified;
context.SaveChanges();

So this works... But, it seemed like a lot of effort, is this the right/best way to achieve it?

I know this is an old question, but I think the problem is timeless. Many to many associations (i.e. without junction class) in Entity Framework are always independent associations, so you can only establish or remove them by manipulating object collections, not primitive key values. Inefficiency is inherent to the implementation.

But it is not prohibited to have a second context that only contains junction tables.

You could create a context that contains the MasterInstanceInstaller junction table and use this to update the associations in the most efficient way you can get using EF:

var installer = installerModel.Installer;

var junctions = context.MasterInstanceInstallers
 .Where(x => x.InstallerId == installer.InstallerId)
 .ToList();

// Delete deselected instances.
foreach(var mi in junctions
 .Where(x => !installerModel.SelectedMasterInstances
 .Contains(x.MasterInstanceId)))

 context.MasterInstanceInstallers.Remove(mi);


// Add newly selected instances.
foreach(int instanceId in installerModel.SelectedMasterInstances
 .Except(junctions.Select(j => j.MasterInstanceId)))
}
 context.MasterInstanceInstallers.Add(new MasterInstanceInstaller
 
 InstallerId = installer.InstallerId,
 MasterInstanceId = instanceId
 
 );
}
context.SaveChanges();

Now, if necessary you can populate the updated many to many association through the main context.

It definitely is timeless. I've done it with and without the explicit junction table over the years but for the first time I find myself wanting the best of both worlds needing both:

1. "mutual collections" approach
   that is in effect by omitting the junction table in the model


2. Explicit junction table in model approach
   Letting one grapple with joins

I'm currently considering using a view to avoid explicit inclusion of junction table in the model in order to let me do the joins I might sometimes need and to also shape the returned data with additional selection criteria in the ON clauses used to join each side of the many-to-many relationship. Will report back as to the success of failure of this approach.