Help setting up a fetchRequest?

[geerlingguy]

I have a navigation-based view setup, where I have a MKMapView in the first view that loads, with a bunch of annotations.

When the annotation's detail disclosure is tapped, I push into a second view display (a detail view), and the annotation passes a title and a parishNumber to the detail view.

What I'd like to do is use that parishNumber to lookup the values for one 'Parish' record in my core data store, using a fetchRequest.

However, my familiarity with Core Data is less than poor, and I was wondering if anyone had any good pointers for doing a query like this against Core Data (something like grabbing one row in SQL with a WHERE clause like "WHERE parishNumber = 123").

I can get the entire array using a fetch request that gets all the objects like the following:

NSArray *fetchedObjects = [context executeFetchRequest:fetchRequest error:&error];
	for (Parish *info in fetchedObjects) {
	// do stuff here
}

...but I don't know how to just grab one 'row' from Core Data. Surely there's a more performant option than getting the entire array again?

[dljeffery]

Take a look at NSPredicate. You would do something like this before calling executeFetchRequest:

NSPredicate *searchPredicate = [NSPredicate predicateWithFormat:@"(parishNumber == %d)", parishNumber];
[fetchRequest setPredicate:searchPredicate];

[geerlingguy]

Originally Posted by dljeffery

Take a look at NSPredicate. You would do something like this before calling executeFetchRequest:

NSPredicate *searchPredicate = [NSPredicate predicateWithFormat:@"(parishNumber == %d)", parishNumber];
[fetchRequest setPredicate:searchPredicate];

Ah, didn't know about NSPredicate — thanks!

On a related note - do you think it would be better to set up one NSArray of NSDictionaries for the entire set of data at the beginning of the class, and use that array to retrieve individual parishes (based on their parishNumbers), or just query Core Data each time?

The reason I ask is that, coming from a PHP background, I know that it's often quicker to keep the data in memory and query it there, than to query an external database more than once... does this apply with the Core Data model, too?

[dljeffery]

If it were me, I think I'd just go with running the queries rather than keeping everything in memory. And then only move to keeping things all in memory if the performs seems to be lagging... but bearing in mind that you might start seeing RAM issues as a result (not sure how large your data set is, though).

Also, SQLite isn't going to have the out-of-process overhead like you'd see querying MySQL or PostgreSQL or whatever from your PHP code.

[geerlingguy]

Thanks for the info - I haven't done too much performance profiling in the app so far, but I might try both routes (I have both working) with some larger datasets (I have one table with over 3,000 rows), and threading, to see if I can optimize the lag time for processing.

Right now it looks like my smaller dataset might perform best if I load it in memory at app launch, and my larger set would be best used by individual queries throughout the app's views.