Question

I am needing to do a “join”-type operation on two tables/models from my Core Data storage, and I’m having a difficult time finding good examples out there for getting something like this to work.

It’s fairly simple what I need to do. I have two models/tables: locations and location_categories, where locations contains a location/establishment’s details (along with a unique ID), and the location_categories table contains category_id’s with associated location_id’s. I need to select all the locations where, given a category_id, it joins the two tables/models. If it was straight SQL, it would look like this (and this query does work when I run it against the actual SQLite DB):

// pretend we passed in a catID of 29:
SELECT * FROM zlocation where zlocid in (select zlocid from zloccategory where zcatid = 29)

So as you can see, it’s pretty straight-forward. Is there a way to do this without having to do two fetches and for-loops? My current objective-c code to do this is below, and it works fine, but of course it’s horribly slow and inefficient, due to a lot of data being in both tables. The for-loops are causing a huge slowdown, and my gut tells me that this work really should be done on the DB/Core-Data side of things:

- (NSMutableArray *) fetchLocsWithCatID:(NSString *)catID {
    NSMutableArray *retArr = [[NSMutableArray alloc] init];

    NSManagedObjectContext *context = [self managedObjectContext];
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    [fetchRequest setEntity:[NSEntityDescription entityForName:MODELNAME_LOCCATEGORIES inManagedObjectContext:context]];
    [fetchRequest setPredicate: [NSPredicate predicateWithFormat:@"catID == %@", catID]];

    NSError *error;
    NSArray *locCats = [context executeFetchRequest:fetchRequest error:&error];
    NSArray *allLocsArr = [self fetchAllDataForTable:MODELNAME_LOCATION]; // this retrieves contents of entire table

    for (Location *currLoc in allLocsArr) {
        for (LocCategory *currLocCat in locCats) {
            if ([currLoc.locID isEqualToString:currLocCat.locID]) {
                if (![retArr containsObject:currLoc]) {
                    [retArr addObject:currLoc];
                }
            }
        }
    }

    return retArr;
}

Any thoughts on whether this is possible with one single predicate/fetch? Or dunno.. maybe there is a more efficient way to join those two NSArrays via the objective-c code?

Thanks in advance for any direction you can offer on this!

Answer 1

The best way would be to use relationships.

This way when you fetch a Location you can also access the LocationCategory like myLocation.locationCategory

You would need to set up your relationsship like so:

(Entity)LocationCategory.(relationship)locations <->> (Entity)Location.(relationship)locationCategory

It’s a to-many relationship. Don’t forget the inverse either.

Once that’s set up you just need to connect relationships up where ever you create the entity instances. Like:

 Location *myLocation = ...
 LocationCategory *myLocationCategory = ...
 myLocation.locationCategoy = myLocationCategory;

Also make sure you are using the same MOC to fetch the objects. I strongly recommend MagicalRecord to easily manage creating objects and fetching in the specified moc.