Fetched Property in Core Data

Question

In my core data model, a Person has one or more Cars, specified by the unordered to-many relationship 'Cars'. Frequently, I need to retrieve a Person's cars ordered by datePurchased, or by dateLastUsed.

Until now, I have been adding my own method to Person for carsByDatePurchased. This uses a sort descriptor to sort the NSSet cars and return an NSArray.

Could/should I instead use a Fetched Property for this? I am experiencing some performance overhead using the sort descriptor every time I need the cars in a certain order, even going so far as implementing my own caching of carsByDatePurchased. It looks like the fetched property is cached for me - is that correct?

What are the limitations of a fetched property vs my own implementation?

And crucially, does the fetched property's value persist between executions? If I update the fetched property and save my context, is the value stored for the next time I launch the application?

Answer 1

A fetched property will work, and indeed I used it in my own project with a Post->Comment relationship which needs to be sorted by 'date added index'.

There are a number of caveats: You cannot specify a sort descriptor in the visual editor and have to specify it in code.

I use something like this

    // Find the fetched properties, and make them sorted...
for (NSEntityDescription *entity in [_managedObjectModel entities])
{
    for (NSPropertyDescription *property in [entity properties])
    {
        if ([property isKindOfClass:[NSFetchedPropertyDescription class]])
        {
            NSFetchedPropertyDescription *fetchedProperty = (NSFetchedPropertyDescription *)property;
            NSFetchRequest *fetchRequest = [fetchedProperty fetchRequest];

            // Only sort by name if the destination entity actually has a "index" field
            if ([[[[fetchRequest entity] propertiesByName] allKeys] containsObject:@"index"])
            {
                NSSortDescriptor *sortByName = [[NSSortDescriptor alloc] initWithKey:@"index"
                                                                           ascending:YES];

                [fetchRequest setSortDescriptors:[NSArray arrayWithObject:sortByName]];
            }
        }
    }
}

In My Post entity I have a fetched property called "sortedComments" which is defined as:

post == $FETCH_SOURCE

where posts have a to-many "comments" relationship and comments have a "post" inverse

In opposition to the other answers here: The benefits of using a fetched property like this, is CoreData takes care of the caching and invalidating the cache as comments for a post or indeed the post that owns them changes.