Question

I am working on a iOS application, the model is very simple (Folders and Documents), but I am having problems for making it working with NSFetchedResultsController.

The app uses a UITableView in a NavigationController for showing a list of folders and documents contained in a folder. The user can browse the folders hierarchy with a drilldown effect. Also, the user can copy/paste folders and documents, so any document or folder could have many parents.

This is my current model:

AbstractElement{
    elementName:string
}

Folder{
    parents<<---->>Folder.folders
    folders<<---->>Folder.parents
    documents<<---->>Document.parents
}

Document{
    numPages:int
    parents<<---->>Folder.documents
}

This model works fine if I fill the UITableView with an NSArray created from the current folder:

NSArray * currentListOfElements = 
      [[self.currentFolder.documents sortedArrayUsingDescriptors:descriptors] 
      arrayByAddingObjectsFromArray:
      [self.currentFolder.folders sortedArrayUsingDescriptors:descriptors]];

The problem is that I need to fill the UITableView from a NSFetchedResultsController, getting folders and documents with a single NSFetchRequest. The solution is something like that:

NSEntityDescription *entity = [NSEntityDescription 
                               entityForName:@"AbstractElement" inManagedObjectContext:self.context];
[fetchRequest setEntity:entity];
[fetchRequest setPredicate:[NSPredicate predicateWithFormat:@"%@ IN self.parents", self.currentFolder]];

...create the NSFetchedResultsController using fetchRequest...

But this doesn’t work because AbstractElement doesn’t have “parents” relationship.

If I move parent relationship from Folder and Document to AbstractElement the problem is how to set the inverse relation:

AbstractElement{
    elementName:string
    parents<<---->> ??
}

Folder{
    folders<<---->>Folder.parents
    documents<<---->>Document.parents
}

Document{
    numPages:int
}

Is this the right aproach? How could I use NSFetchedResultsController with my model? I don’t want to remove inverse relations because of the extra code that I need to implement.

Thanks.

Answer 1

Louissmr,

You say,

… the user can copy/paste folders and documents, so any document or
folder could have many parents.

Hence, What is wrong with having 2 to-many relations in the AbstractElement. Call them, say, parents and children. It meets your criteria. Each is the inverse of the other.

Andrew