Question

An image has many galleries, and a gallery has many images. I am trying to update the images that a gallery has. The code below works, but feels somewhat clumsy.

   var g=db.Galleries.Find(gal.Id);

   var ims = gal.Images.Select(i => db.Images.Where(im => im.Id == i.Id && im.User.Id == user.Id)).SelectMany(im => im).ToList();

   g.Name = gal.Name;

   g.Images.Clear();

   foreach (var im in ims)
   {
        g.Images.Add(im);
   }

   db.SaveChanges();

When I do this:

g.Images=ims;

instead of:

   g.Images.Clear();

   foreach (var im in ims)
   {
        g.Images.Add(im);
   }

An exception is thrown:

Violation of PRIMARY KEY constraint 'PK_GalleryImages'. Cannot insert duplicate key in object 'dbo.GalleryImages'.

Can you explain why? Is there a better way to approach this?

Answer 1

When you do g.Images.Add(im), entity framework will add a new row with galleryId and imageId in dbo.GalleryImages table if it doesn’t already exist in the table.

On the otherhand, when you do g.Images=ims, you are actually telling entity framework to add a new relation (new row in GalleryImages table). If you were to add new images into the gallery g.Images=ims would work fine.