Question

Working on an app where I have a large collections of managed objects against which I want to fetch a few random instances.

My question is, is there any way I can use NSPredicate and NSFetchRequest to return several objects at random.

I saw that you could actually add a NSFetchRequest into the entity using the data modeler, any way to do the random fetch using this?

Also what would be the best method for determining the “count” of a table so I can set the bounds of the random number generator.

let me know if you need more details.

Thanks!

Nick

Answer 1

This may not be exactly how you implement this, but hopefully it will get you started.

Somewhere in your header or at the top of your implementation file:

#import <stdlib.h>
#import <time.h>

Elsewhere in your implementation:

//
// get count of entities
//
NSFetchRequest *myRequest = [[NSFetchRequest alloc] init];
[myRequest setEntity: [NSEntityDescription entityForName:myEntityName inManagedObjectContext:myManagedObjectContext]];
NSError *error = nil;
NSUInteger myEntityCount = [myManagedObjectContext countForFetchRequest:myRequest error:&error];    
[myRequest release];

//
// add another fetch request that fetches all entities for myEntityName -- you fill in the details
// if you don't trigger faults or access properties this should not be too expensive
//
NSArray *myEntities = [...];

//
// sample with replacement, i.e. you may get duplicates
//
srandom(time(NULL)); // seed random number generator, so that you get a reasonably different series of random integers on each execution
NSUInteger numberOfRandomSamples = ...;
NSMutableSet *sampledEntities = [NSMutableSet setWithCapacity:numberOfRandomSamples];
for (NSInteger sampleIndex = 0; sampleIndex < numberOfRandomSamples; sampleIndex++) {
    int randomEntityIndex = random() % myEntityCount; // generates random integer between 0 and myEntityCount-1
    [sampledEntities addObject:[myEntities objectAtIndex:randomEntityIndex]];
}

// do stuff with sampledEntities set

If you need to sample without replacement, to eliminate duplicates, you might create an NSSet of randomEntityIndex NSNumber objects, instead of just sampling random ints.

In this case, sample from an ordered NSSet, remove NSNumber objects as you pull them out of the bag, and decrement myEntityCount for the purposes of picking a random NSNumber object from the set.