在实体框架CF中删除相关项目

Question

I'm trying to remove an item from database,

This item is related to other items. when i try to remove it like this.

using (var ctx = new UmeaKommunFacilityContext())
                        {
                            try
                            {
                                ctx.Facility.Attach(oldFacilitiesToDelete[i]);
                                ctx.Facility.Remove(oldFacilitiesToDelete[i]);
                                ctx.SaveChanges();
                            }
                            catch (Exception e)
                            { 

                            }
                        }

then i'm getting

An entity object cannot be referenced by multiple instances of IEntityChangeTracker

i added this code to detach the item

ctx.Entry(oldFacilitiesToDelete[i]).State = EntityState.Detached;
                        ctx.Facility.Attach(oldFacilitiesToDelete[i]);
                        ctx.Facility.Remove(oldFacilitiesToDelete[i]);
                        ctx.SaveChanges();

then i got the same exception.

then what to do to be able to remove the item from db. this item type (Facility) is related to other types, and i have removed all aother related by this code and it worked

  using (var db2 = new UmeaKommunFacilityContext())
                {
                    try
                    {

                        Facility f = oldFacilitiesToDelete[i];
                        List<Image> images = f.Images.ToList<Image>();
                        for (int j = 0; j < images.Count; j++)
                        {
                            db.Images.Attach(images[i]);
                            db.Images.Remove(images[i]);
                            db.Entry(images[i]).State = EntityState.Detached;

                        }


                        List<Material> materials = f.Materials;
                        for (int j = 0; j < materials.Count; j++)
                        {
                            db.Materials.Attach(materials[i]);
                            db.Materials.Remove(materials[i]);
                            db.Entry(materials[i]).State = EntityState.Detached;

                        }


                        List<Polygon> polygons = f.Polygons;
                        for (int j = 0; j < polygons.Count; j++)
                        {
                            db.Polygons.Attach(polygons[i]);
                            db.Polygons.Remove(polygons[i]);
                            db.Entry(polygons[i]).State = EntityState.Detached;
                        }


                        db.SaveChanges();

                    }
                    catch (EntityException ex)
                    {
                    }
                    catch (Exception e)
                    {  }

                }

Answer 1

Why are you doing Attach() and Remove() on the same item?

To remove an item from database you need to use the Delete action. For example:

ctx.Entry(oldFacilitiesToDelete[i]).State = EntityState.Deleted;
ctx.SaveChanges();

When you "Detach" an item you're simply removing it from the context, not from the database. That why you can always reverse with "Attach".

If the point here is to remove the related entity for an items, it's easier to implement .OnCascadeDelete(true) so you can remove only the main entity; EF will remove the other related items for you, like this:

        HasRequired(t => t.MainEntity)
          .WithMany(t => t.SubEntity)
          .HasForeignKey(d => d.MainEntityId)
          .WillCascadeOnDelete(true);