在 ASP.NET Core MVC 中从 ViewModel 更新 ICollection

Question

I am using ASP.NET Core MVC and EF6, and I am having trouble updating a many-to-many relationship. My model has an Asset table and a Group table. The relationship between Assets and Groups is set as many-to-many. Thus far, I have the following code that is able to assign groups to the assets through the AssetGroup table that EF automatically created. It works when adding either a single group or multiple groups.

The problem is that when I want to update the assigned groups, it fails to properly remove old groups.

Below is my code.

Models :

public class Asset : BaseEntity
{
    public Asset()
    {
        this.Group = new HashSet<Group>();
    }

    [Required]
    public string Name { get; set; }
    public string? Description { get; set; }
    public ICollection<Group>? Group { get; set; }
}

public class Group : BaseEntity
{
    public Asset()
    {
        this.Asset = new HashSet<Asset>();
    }

    [Required]
    public string Name { get; set; }
    public ICollection<Asset>? Asset { get; set; }
}

DbContext.cs:

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    //new DBDesigner().ConfigureModels(modelBuilder);
    base.OnModelCreating(modelBuilder);

    // Many-to-many relationship with assets and groups
    modelBuilder
        .Entity<Asset>()
        .HasMany(p => p.Group)
        .WithMany(p => p.Asset)
        .UsingEntity(j => j.ToTable("AssetGroup"));
}

AssetCreateVM.cs:

public class AssetCreateVM
{
    public int Id { get; set; }

    [Required]
    public string Name { get; set; }
    public string? Description { get; set; }
    public List<int>? GroupIds { get; set; }
    public SelectList? Groups { get; set; }
}

AssetsController.cs:

public async Task<IActionResult> Edit(int id, AssetCreateVM viewModel)
{
    if (ModelState.IsValid)
    {
        var model = mapper.Map<Asset>(viewModel);

        // Clear assigned groups from an asset if viewModel.GroupIds is empty
        if (viewModel.GroupIds == null)
        {
            // Remove existing group from the AssetGroup table
            model.Group.Clear();
            await assetRepository.UpdateAsync(model);
        }

        // Add new groups to AssetGroup table
        // Loop through the groupId
        foreach (var item in viewModel.GroupIds)
        {
            // Add group to the AssetGroup table
            var group = await groupRepository.GetAsync(item);
            if (group != null) model.Group.Add(group);
        }

        if (model.Group != null)
        {
            await assetRepository.UpdateAsync(model);
        }

        return RedirectToAction(nameof(Index));
    }

    viewModel.Groups = new SelectList(await groupRepository.GetAllAsync(), "Id", "Name", viewModel.GroupIds);

    return View(viewModel);
}

This is the UpdateAsync within the GenericRepository.cs

public async Task UpdateAsync(T entity)
{
    _context.Update(entity);
    await _context.SaveChangesAsync();
}

As stated, the above code work as expected when adding either single or multiple groups to an asset. However, when I want to remove the assigned groups, the code no longer works.

The problem is in the following code:

if (viewModel.GroupIds == null)
{
    // Remove existing group from the AssetGroup table
    model.Group.Clear();
    await assetRepository.UpdateAsync(model);
}

It seems like the model.Group.Clear() is clearing out the Groups from Asset however, the UpdateAsync is not applying the changes to the database correctly. ie the rows are not being removed from the AssetGroup table.

Why is the AssetGroup changes not being updated when I remove the group association?

Answer 1

you should use RemoveRange or Remove instead Update for delete item

Context.Assets.Group.RemoveRange(entities) // entites like model.Group

or

Context.Group.RemoveRange(entities)

If you can't access removerange you can do it as follows

model.Group.ForEach(group => repo.Delete(group));