使用实体框架从数据库中删除相关对象

Question

This is my first question on this site. I am teaching myself MVC and Entity Framework in an ASP.Net web app. I have a series of related entities and when I call context.saveChanges() the removed related data items do not get removed.

My Entities I have an Item Entity that can have 0 to Many Aliases. This was database first project and most of these lines of code have been auto generated.

public Class Item
{
  public int Id{get;set};
  public string Name {get;set;}
  public virtual ICollection<AliasClass> AssociatedAliases {get;set;}
}

public Class AliasClass
{
  public int Id {get;set;}
  public int ItemId {get;set;}
  public string AliasName {get;set;}

  public virtual Item RelatedItem {get;set;}
}

Ok, we are done with the autogenerated stuff. My Postback action accepts a postback viewmodel. I am implementing repository pattern. And my Item Repo has an update function.

[HttpPost]
public ActionResult Edit (Models.EditViewPostBack MyPostBackModel)
{
  If (!ModelState.isValid){
    ... rebuild and copy  // this part works. Skipping over it
  }

  // this may be a key line of my issue
  MyPostBackModel.Item.Alias = MyPostBackModel.myAliases;
  ItemRepo.Update(MyPostBackModel.Item);
  return RedirectToAction("Index","Home");
}

Here is Models.EditViewPostBack. The new list of associations get passed back as an array. New associations have an id of 0. At update it will be assigned a real Id.

    public class EditViewPostBack{
      Item myItem {get; set;}
      AliasClass[] myAliases {get;set;}
    }

Now, finally the meat of the project and my real issue. This is the method from the ItemRepo that is supposed to update changes.

    public void Update (Item myItem)
    {

      // myItem.AssociatedAliases contains only the wanted associations. 
      // It doesn't have the to-be deleted items. 
      // new items have an id of 0

      // defined in class constructor. 
      context.Item.Attach(myItem);
      context.Entry(Item).State = EntityState.Modified;
      foreach(var name in myItem.AssociatedAliases){
        // finds the modified local version of the item. doesn't connect
        // to the database to location info.
        if(name.id > 0)
        {
          var moded = context.AliasClass.find(name.Id); 
          context.Entry(moded).State = EntityState.Modified;
        }
        else
        {
          // it is brand new
          context.Item.Add(name)
        }
      }          
      //
      // Once the alias is not attached to it's item it has to go
      // How to handle deletes goes here
      //
      context.SaveChanges();
    }

Becuase the entitystate isn't marked as deleted the association remains and the item appears like I never removed it. I have already tried several things.

If i load or search the for the original item with a context.Alias.linqFunction()the entire list is re-associated with the myItem parameter since the id's match.

// invoking the following line causes Item.Alias to fully re-associate.
// I lose the list passed to the function 
context.AliasClass.Where(x=>x.ItemId == myItem.Id)

I currently cannot load an Alias because that would be a round trip to the database where I am not deleting the item.

I think I need a way to Remove except where Id's match the still connected list. But remove takes an instance of the item to remove.

What can I do to delete these associated Alias names from my database?

Answer 1

Your issue is because of having three situation for Aliases . They maybe are new and or edited or mixed. So for handling that you first need to handle Aliases change separated than Item For doing this create new Alias[] and then do this:

    foreach(var name in myItem.Aliases){
             if it is edited one: update it
             if it is new one: save it with id of myItem
             }

Then before updating myItem , just put:

    myItem.Aliases=null;

If you have some deleted Aliases , you can't count on EF inelegance to handle deleting them as you so on adding new ones. You need to delete them by yourself. So should have something like

allAliases=context.Getall(Aliases).where(they are for current item);

Then you can have

foreach(var aliase in allAliases){
         if it it isnot in myItem.Aliases: delete it
         }

For better performance and less complexity, I suggest you first delete all Aliases in db and then all your myItem.Aliases will be as new ones. So when ever you save your myItem all related Aliases will be added automatically and you have just two times interaction with db which means better performance.