iPhone CoreData Queries

I'm new at using CoreData and I'm trying to understand how to perform a query on a table. I can use开发者_StackOverflow社区 a fetch request to pull all of the records from a table, but I'm looking for a subset. Is there an easy way to do this?

Thanks, Howie

Have you looked into Predicates?

Also, buy Marcus Zarra's book on Core Data.

You can add a NSPredicate to the NSFetchRequest to filter the records that are returned. You can also control what is populated in the returned objects (only populate properties, include relationships, populate nothing, just return a count, etc.) but as Peter pointed out, Core Data is an object hierarchy and model API that just happens to store to a database. It is far easier to work with when you look at it from that POV.

You have to do something like:

// Init your fetchRequest
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];

NSEntityDescription *entityDescription = [NSEntityDescription entityForName:@"entityName" inManagedObjectContext:yourManagedObjectContext];

// create the relation between request and the created entity
[fetchRequest setEntity:entityDescription]; 

// Set your predicate for this request
// For more info take a look at NSPredicate Class Reference
// http://developer.apple.com/mac/library/DOCUMENTATION/Cocoa/Reference/Foundation/Classes/NSPredicate_Class/Reference/NSPredicate.html
[fetchRequest setPredicate:somePredicate];

// Pushing the results into a mutable array
NSMutableArray *mutableFetchResults = [[yourManagedObjectContext executeFetchRequest:fetchRequest error:&error] mutableCopy];

[fetchRequest release];