I am trying to build an iTunes-like data browser for my bibliographic app. My app is based on a core data model: book <—» passage «—» topics. The databrowser is made up of three columns, bind to 3 NSArrayController; these controllers are bind to the managedObjectContext; I am using the contentSet binding to link every controller to the one preceding it; this produces the very same behaviour that is observed in iTunes when one selcts a genre, then artist, etc. Here is where the indirect relationship comes on the way. My data browser follows this sequence: topic –> author –> book. The column sequenced is obvioulsy mirrored by the array controller sequence: Topic NSArrayController - Book NSArrayController - Passage NSArrayController. And here is the trouble. Of course the relationship between books and topics is only indirect; they are only associated through the passage entity. How do I bind the Book NSArrayController to get only books associated to passages associated to the selected topic of the Topic NSArrayController? I hope I have been clear enough. Thanks in advance, Davide —- If your relationships are bidirectional, you could use a key path. For instance, if you have one topic selected called aTopic, you would get the books with
NSArray *books = [aTopic valueForKeyPath:@"passages.book"];
Thanks for the suggestion; unluckily it doesn’t work. Here is the error log:
2005-09-13 09:51:26.596 BiblioBeta *** NSRunLoop ignoring exception ‘[<_NSFaultingMutableSet 0x398b60> addObserver:forKeyPath:options:context:] is not supported. Key path: entry’ that raised during posting of delayed perform with target 4e45df0 and selector ‘invokeWithTarget:’
(entry is the real name of the book entity; i just used book to make things clearer)
I believe that you would want to bind contentObject and contentArrayForMultipleSelection in the Books NSArrayController to a key path like
selection.@distinctUnionOfArrays.books in the Passages NSArrayController.
I managed to solve the problem, partially, this way: I bind the Entry (that is books) NSArrayController contentSet to selection.passages and the column of my table in the interface to book.authors. Now, this works, but as you said I get duplicates: if more passages of the same book are associated to a topic, the author name gets duplicated in the column. I tried to bind the column to book.@distinctUnionOfArrays.author, but the table stops working and the log says:
2005-09-14 10:53:42.431 BiblioBeta [<BibliographicEntry 0x3182a0> valueForUndefinedKey:]: this class is not key value coding-compliant for the key @distinctUnionOfArrays.
For the sake of clearness I set up a simple page to show my app structure: http://www.cnomania.it/garage/appreference.htm
CoreData uses sets, not arrays. Try @distinctUnionOfSets. –JediKnil —- In the cocoadev list John Timmer gave me the right hint. @distinctUnionOfSets and @distinctUnionOfArrays are read only, while KVC requires keys to be also able to write. As a solution I considered the option of subclassing NSManagedObject to a method that reads @distinctUnionOfSets and writes (void). Yet I realized that I must think carefull about the way in which data are filtered by the NSArrayController, to make the columns really work.
Example: If I choose the Topic of “love” in the first column and Shakespeare in the second column, I must makke sure that the third column shows only those works by shakespeare whose passages are associated to love. If I am not careful I could end up showing all the works by shekespeare, so the first column become useless.
I came to the conclusion that I must implement a separate author entity (this also to allow inserting books with more than one author) and find a way to associate topics also to the Entry (which is book) entity, maybe through a fetched property. Do you think there is a way, a straight one, to add to the Entry (book) entity a property that is an array of all the Topics associated to its Passages? Cheers, DavideBenini