多对多插入或更新实体框架

Question

I am trying to update a join table using Entity Framework that has the following design

I am trying the following piece of code but it doesn't update the join table DeliveryDayBrands, please tell me what I am doing wrong. Thanks.

public async Task<Brand> Update(Brand brand, IEnumerable<int> SelectedDeliveryDays)
    {

        using (var dc = DataContext())
        {
            foreach (int deliveryDayId in SelectedDeliveryDays)
            {
                DeliveryDay deliveryDay = new DeliveryDay();
                deliveryDay = dc.DeliveryDays.FirstOrDefault(d => d.DeliveryDayId == deliveryDayId);


                brand.DeliveryDays.Add(deliveryDay);

            }

            dc.SetModified(brand);
            await dc.SaveChangesAsync();
            return brand;
        }
    }

Answer 1

It looks like you're trying to add some already existing DeliveryDays to a Brand. You don't need to re-add it to the database, as doing so will result in creating duplicates.

Also, as described in this answer , DataContext is used for LINQ to SQL. The answer says you need to use the DbContext class to talk to Entity Framework.

I suggest you try to fetch the connection string to the database from your web.config (or elsewhere), and then try the following:

public async Task<Brand> Update(Brand brand, IEnumerable<int> SelectedDeliveryDays)
{
    using (var context = new DbContext([insert connectionString to database here]))
    {
        foreach (int deliveryDayId in SelectedDeliveryDays)
        {
            DeliveryDay deliveryDay = context.DeliveryDays.FirstOrDefault(
               d => d.DeliveryDayId == deliveryDayId);

            deliveryDay.Brands.Add(brand);
            brand.DeliveryDays.Add(deliveryDay)

            // You could also use your custom SetModified method.
            context.Entry(brand).State = EntityState.Modified;
            context.Entry(deliveryDay).State = EntityState.Modified;
        }            

        await context.SaveChangesAsync();
        return brand;
    }
}