EF ComboBox Does Not Display SelectedItem Binding

Question

I use Entity Framework and Caliburn.Micro to implement an MVVM application.

Basically, I created AuthorModel and BookModel in a one to many relationship - to an Author who has several books and books that have only one author.

I have a SelectBookWindow where I use a DbContext to load an ObservableCollection<Book> , from where I select the book that I want to view / edit. Then I pass the selected book as a parameter to EditBookWindow, where I have a combobox listing all the authors, but with the selected author.

Here I load the ObservableCollection<Author> using another DbContext instance and set it as combobox ItemsSource , as well as do SelectedItem="{Binding Author}" . ( Author is a virtual property of Book )

ComboBox displays the list of authors correctly. However, it does not appear to display the Book Author as a SelectedItem.

Is it because I'm using another instance of DbContext? If so, how can I fix this problem?

Answer 1

Yes it is. Since the author in the ItemsSource refers to another object, although the content is the same as the one that is attached to the SelectedItem.

I know little about EF, I think you can use the same context for two objects. Or override the equals (and gethashcode) of the Author class to compare the contents, then return true if it is.

Answer 2

As Eben mentions, the Author referenced by the ItemsSource will be a different object (although it is related to the same object).

I think your approach to using the new DbContext for both of your windows is correct, you may run into problems if you have persistent / shared DbContexts ; it makes sense to load EditWindow into a new context, make changes and get rid of the context.

One option is to Detach your Book object from the old DbContext and Attach into a new context: (a good explanation here is Entity Framework 4 - AddObject vs Attach ).

I would probably just approve of using the passed Book object to reload the selected Book from the new context, for example. using DbSet<TEntity>.Find and using this restored object to bind to SelectedItem (and not to the one you go through windows).