Entity framework core update many to many

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This will work for you.

Make a class to have the relationship in:

public class ColorProduct
{
    public int ProductId { get; set; }
    public int ColorId { get; set; }

    public Color Color { get; set; }
    public Product Product { get; set; }
}

Add a ColorProduct collection to your Product and Color classes:

 public ICollection<ColorProduct> ColorProducts { get; set; }

Then use this extension I made to remove the unselected and add the newly selected to the list:

public static void TryUpdateManyToMany<T, TKey>(this DbContext db, IEnumerable<T> currentItems, IEnumerable<T> newItems, Func<T, TKey> getKey) where T : class
{
    db.Set<T>().RemoveRange(currentItems.Except(newItems, getKey));
    db.Set<T>().AddRange(newItems.Except(currentItems, getKey));
}

public static IEnumerable<T> Except<T, TKey>(this IEnumerable<T> items, IEnumerable<T> other, Func<T, TKey> getKeyFunc)
{
    return items
        .GroupJoin(other, getKeyFunc, getKeyFunc, (item, tempItems) => new { item, tempItems })
        .SelectMany(t => t.tempItems.DefaultIfEmpty(), (t, temp) => new { t, temp })
        .Where(t => ReferenceEquals(null, t.temp) || t.temp.Equals(default(T)))
        .Select(t => t.t.item);
}

Using it looks like this:

var model = _db.Products
    .Include(x => x.ColorProducts)
    .FirstOrDefault(x => x.ProductId == vm.Product.ProductId);

_db.TryUpdateManyToMany(model.ColorProducts, vm.ColorsSelected
    .Select(x => new ColorProduct
    {
        ColorId = x,
        ProductId = vm.Product.ProductId
    }), x => x.ColorId);

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