Entity framework core update many to many

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);

In order to avoid the LINQ hell in the above answer, the templated "Except" method can be rewritten as such:

public static IEnumerable<TEntity> LeftComplementRight<TEntity, TKey>(
        this IEnumerable<TEntity> left,
        IEnumerable<TEntity> right,
        Func<TEntity, TKey> keyRetrievalFunction)
    {
        var leftSet = left.ToList();
        var rightSet = right.ToList();

        var leftSetKeys = leftSet.Select(keyRetrievalFunction);
        var rightSetKeys = rightSet.Select(keyRetrievalFunction);

        var deltaKeys = leftSetKeys.Except(rightSetKeys);
        var leftComplementRightSet = leftSet.Where(i => deltaKeys.Contains(keyRetrievalFunction.Invoke(i)));
        return leftComplementRightSet;
    }

Furthermore the UpdateManyToMany method can be updated to include entities that have been modified as such:

public void UpdateManyToMany<TDependentEntity, TKey>(
        IEnumerable<TDependentEntity> dbEntries,
        IEnumerable<TDependentEntity> updatedEntries,
        Func<TDependentEntity, TKey> keyRetrievalFunction)
        where TDependentEntity : class
    {
        var oldItems = dbEntries.ToList();
        var newItems = updatedEntries.ToList();
        var toBeRemoved = oldItems.LeftComplementRight(newItems, keyRetrievalFunction);
        var toBeAdded = newItems.LeftComplementRight(oldItems, keyRetrievalFunction);
        var toBeUpdated = oldItems.Intersect(newItems, keyRetrievalFunction);

        this.Context.Set<TDependentEntity>().RemoveRange(toBeRemoved);
        this.Context.Set<TDependentEntity>().AddRange(toBeAdded);
        foreach (var entity in toBeUpdated)
        {
            var changed = newItems.Single(i => keyRetrievalFunction.Invoke(i).Equals(keyRetrievalFunction.Invoke(entity)));
            this.Context.Entry(entity).CurrentValues.SetValues(changed);
        }
    }

which uses another custom templated "Intersect" function to find the intersection of the two sets:

public static IEnumerable<TEntity> Intersect<TEntity, TKey>(
        this IEnumerable<TEntity> left,
        IEnumerable<TEntity> right,
        Func<TEntity, TKey> keyRetrievalFunction)
    {
        var leftSet = left.ToList();
        var rightSet = right.ToList();

        var leftSetKeys = leftSet.Select(keyRetrievalFunction);
        var rightSetKeys = rightSet.Select(keyRetrievalFunction);

        var intersectKeys = leftSetKeys.Intersect(rightSetKeys);
        var intersectionEntities = leftSet.Where(i => intersectKeys.Contains(keyRetrievalFunction.Invoke(i)));
        return intersectionEntities;
    }