iPhone OS: Fetching a random entity instance using NSPredicate Nsfetchrequest and core data

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

Instead use the fetchLimit in conjunction with the fetchOffset that way you can efficiently fetch only a single entity into memory:

NSFetchRequest *myRequest = [[NSFetchRequest alloc] init];
[myRequest setEntity: [NSEntityDescription entityForName:myEntityName inManagedObjectContext:myManagedObjectContext]];
NSError *error = nil;
NSUInteger myEntityCount = [myManagedObjectContext countForFetchRequest:myRequest error:&error];    

NSUInteger offset = myEntityCount - (arc4random() % myEntityCount);
[myRequest setFetchOffset:offset];
[myRequest setFetchLimit:1];

NSArray* objects = [myManagedObjectContext executeFetchRequest:myRequest error:&error];    
id randomObject = [objects objectAtIndex:0];

Answer 2

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.

Answer 3

I've searched around a lot for this, essentially Coredata won't give you any random rows, and it is not meant to. You have to create your own.

This is what I came up with up with, assuming we are using an NSPredicate and theres no primary Unique key, this is the best possible answer i think with the least overhead.

1. Set NSFetchRequest type to NSManagedObjectID. Turn everything off, to minimize overhead.
2. Execute fetch request with your desired predicate, Do Not use any FetchLimit.

3. From the received NSManagedObjectID array. get your random number of objects. this is a good solution: Get n random objects (for example 4) from nsarray

4. Now you've got random NSManagedObjectIDs of your desired count, (which are more or less random)

5. Loop through the random objectID array and Use NSManagedObjectContext objectWithID: to get the objects.

Answer 4

If you are fetching all the objects anyway, there is no need for the first request to get the object count. You can just use something like:

myEntityCount = [myEntities count]

Answer 5

Solution suggested by Core will not work if you need to randomize the fetch within a subset table rows restricted by a predicate (e.g. "where something < something").

The best solution I have so far (where I do not need to fetching all or large number of rows) is using randomly selecting based on a primary key (off course the requires a primary key in the table, preferably w/o any missing values).