NSFetchedResultsController with relationship not updating

ghz 7months ago ⋅ 55 views

Let's say I have two entities, Employee and Department. A department has a to-many relationship with an employee, many employees can be in each department but each employee only belongs to one department. I want to display all of the employees in a table view sorted by data that is a property of the department they belong to using an NSFetchedResultsController. The problem is that I want my table to update when a department object receives changes just like it does if the regular properties of employee change, but the NSFetchedResultsController doesn't seem to track related objects. I've gotten passed this issue partially by doing the following:

for (Employee* employee in department.employees) {
    [employee willChangeValueForKey:@"dept"];
}

/* Make Changes to department object */

for (Employee* employee in department.employees) {
    [employee didChangeValueForKey:@"dept"];
}

This is obviously not ideal but it does cause the employee based FRC delegate method didChangeObject to get called. The real problem I have left now is in the sorting a FRC that is tracking employee objects:

NSEntityDescription *employee = [NSEntityDescription entityForName:@"Employee" inManagedObjectContext:self.managedObjectContext];
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"department.someProperty" ascending:NO];

This works great and sorts the employees correctly the first time it's called, the problem is that when I make changes to some property of a department that should change the sorting of my employee table, nothing happens. Is there any nice way to have my employee FRC track changes in a relationship? Particularly I just need some way to have it update the sorting when the sort is based on a related property. I've looked through some similar questions but wasn't able to find a satisfactory solution.

Answers

You're facing a common challenge when using NSFetchedResultsController to display data based on relationships in Core Data. By default, NSFetchedResultsController only observes changes to the entity it's configured for, not changes to related entities. However, there are some strategies you can employ to address this issue:

1. Denormalization:

One approach is to denormalize your data model by duplicating the relevant properties from the Department entity into the Employee entity. This allows you to sort the Employee objects directly without relying on relationships. However, denormalization can lead to data redundancy and potential inconsistencies if not managed carefully.

2. KVO Observing:

You can manually observe changes to the related Department objects using Key-Value Observing (KVO). When a Department object changes, you can trigger updates to the Employee objects that are affected by the change. This is similar to the approach you've already tried but can be more robust if implemented correctly.

3. Custom Sorting:

If your sorting criteria are based on dynamic or complex calculations, you may need to implement custom sorting logic instead of relying on NSSortDescriptor. You can fetch the Employee objects and then sort them programmatically based on the properties of their related Department objects.

Example Using KVO Observing:

// Add observer for changes to the department property of Employee
for (Employee *employee in department.employees) {
    [employee addObserver:self forKeyPath:@"department" options:NSKeyValueObservingOptionNew context:nil];
}

// Implement observer method
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
    if ([keyPath isEqualToString:@"department"]) {
        // Handle changes to the department property
        // Update the sorting of your NSFetchedResultsController as needed
    }
}

Note:

  • When using KVO, make sure to properly remove observers when they are no longer needed to avoid memory leaks and unexpected behavior.
  • Ensure that your Core Data relationships are properly set up with appropriate inverse relationships to maintain data integrity and consistency.

By applying one or more of these strategies, you can achieve dynamic sorting based on related properties while still benefiting from the automatic updates provided by NSFetchedResultsController.

#objective-c