Getting or attaching an entity

Question

I have the following method:

public bool RemoveBookCategories(IDictionary<Books, IList<C_Category>> books)
    {
        _context.Configuration.AutoDetectChangesEnabled = true;

        foreach (var book in books.Keys)
        {
            foreach (var category in books[book])
            {
                if (!_context.ChangeTracker.Entries<Books>().Any(e => e.Entity.BookId == book.BookId))
                    _context.Books.Attach(book);
                if (!_context.ChangeTracker.Entries<C_Category>().Any(e => e.Entity.Id == category.Id))
                    _context.C_Category.Attach(category);

                book.C_Category.Remove(category);
            }
        }

        if (_context.SaveChanges() > 0)
            return true;

        return false;
    }

It works as expected.. Sometimes. Other times I get this error message:

{"Attaching an entity of type 'DataAccess.Plusbog.C_Category' failed because another entity of the same type already has the same primary key value. This can happen when using the 'Attach' method or setting the state of an entity to 'Unchanged' or 'Modified' if any entities in the graph have conflicting key values. This may be because some entities are new and have not yet received database-generated key values. In this case use the 'Add' method or the 'Added' entity state to track the graph and then set the state of non-new entities to 'Unchanged' or 'Modified' as appropriate."}

This even though I feel that's pretty much what I try to avoid looking for the entity in the changetracker. I feel like I've tried every solution I could find, but nothing works :-/

Any help would be appreciated :-)

You might have a look at my answer on [ASP.NET MVC - Attaching an entity of type 'MODELNAME' failed because another entity of the same type already has the same primary key value](http://stackoverflow.com/questions/23201907/asp-net-mvc-attaching-an-entity-of-type-modelname-failed-because-another-ent/39557606#39557606). — Murat Yıldız, Sep 18 '16 at 12:28

score 0 · Answer 1 · answered Jan 28 '16 at 10:21

0

Examine closely navigation properties of the entities you're attaching: categories might have Books property which have other books which in their turn have other categories. EF will traverse the whole graph and attach every entity it can reach creating the aforementioned problem. Try blanking some navigation properties which you don't need processed.

I bet you are having this exception while saving the list of books which have one or more categories in common.

answered Jan 28 '16 at 10:21

Maxim Balaganskiy

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Absolutely. The books do have categories in common. I just don't know how to save the categories then, Because I will have to attach them if they're not yet attached, otherwise they're not saved at all. – Nicholas Magnussen Jan 28 '16 at 10:26
I think `book.Categories = null` and `category.Books = null` before attaching should help. – Maxim Balaganskiy Jan 28 '16 at 10:29
Sadly, no dice. Getting the same error when trying to attach. – Nicholas Magnussen Jan 28 '16 at 10:32
Did you anywhere use AsNoTracking to fetch categories using _context? – Maxim Balaganskiy Jan 28 '16 at 10:35
Nope. Through includes, but without AsNoTracking – Nicholas Magnussen Jan 28 '16 at 10:39
Those categories, are they all existing records or there might be new ones? – Maxim Balaganskiy Jan 28 '16 at 10:39
Let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/101869/discussion-between-maxim-balaganskiy-and-nicholas-magnussen). – Maxim Balaganskiy Jan 28 '16 at 10:41

score 0 · Accepted Answer · answered Jan 28 '16 at 11:46

I solved this by attaching my models to a freshly instantiated context and then saving the changes to that one. This points to the fact that I'm retrieving data about books and categories using different contexts and then trying to save the changes to yet another context.

Getting or attaching an entity

2 Answers2