Question

I’ve just started playing with the iOS development and Core Data technology.

What I am trying to do is to insert object into the Core Data and then run fetch request within same context looking for the newly inserted object.

Here is the code I am using to insert the object:

+(Reward*) rewardForAction: (Actions) action
    inManagedObjectContext: (NSManagedObjectContext *) context {

    NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"Reward"];
    NSPredicate *actionPredicate =
        [NSPredicate predicateWithFormat:@"action = %@",
         [NSString stringWithFormat:@"%i", action]];

    NSPredicate *todayPredicate =
        [NSPredicate predicateWithFormat:@"when > %@",
         [NSDate today]];

    NSLog(@"Today's midnight :%@", [NSDate today]);
    request.predicate =
        [NSCompoundPredicate andPredicateWithSubpredicates:
         [NSArray arrayWithObjects: actionPredicate, todayPredicate, nil]];

    NSError *error = nil;
    NSArray *matches = [context executeFetchRequest: request error: &error];

    Reward *reward = nil;
    if(!matches) {
        NSLog(@"ERROR: failed to retrieve rewards");
    } else if(matches.count > 0) {
        NSLog(@"WRONG: reward already exists");
    } else {
        reward = [NSEntityDescription insertNewObjectForEntityForName:@"Reward"
                                               inManagedObjectContext:context];
        reward.action = [NSNumber numberWithInt:action];
        reward.when = [NSDate date];
        reward.pointsEarned = [Reward getPointsForAction:action];
    }

    [Stats track: context];

    [context save:nil];

    return reward;
}

This code first checks if reward is already given today for some action and if not it gives a reward.

calling this method twice with same parameters, like this

[Reward rewardForAction:APPLICATION_LAUNCH
inManagedObjectContext: self.db.managedObjectContext];
[Reward rewardForAction:APPLICATION_LAUNCH
inManagedObjectContext: self.db.managedObjectContext];

expected to result in inserting only one object.

But, in fact it inserts two objects into the Core Data. In the debugger I see second fetch request returns no objects when one is already there.

Looks like NSFetchRequest does not see changes in the data store and operating will on old data. Am I missing something ?

EDIT:
I have also set up observer for the NSManagedObjectContextObjectsDidChangeNotification and when it gets called I am calculating sum of the points to reflect it on the UI. Unfortunately sum also does not contain points from the inserted object. Here is my code to calculate sum:

+(NSInteger) getPoints: (NSManagedObjectContext *) context {
    Stats * stats= [Stats get: context];

    NSArray *args = [NSArray arrayWithObject:
                     [NSExpression expressionForKeyPath:@"pointsEarned"]];
    NSExpression *ex = [NSExpression expressionForFunction:@"sum:"
                                                 arguments:args];

    NSExpressionDescription *ed = [[NSExpressionDescription alloc] init];
    [ed setName:@"result"];
    [ed setExpression:ex];
    [ed setExpressionResultType:NSInteger32AttributeType];

    NSFetchRequest *request = [[NSFetchRequest alloc] init];
    [request setPropertiesToFetch:[NSArray arrayWithObject:ed]];
    [request setResultType:NSDictionaryResultType];

    /*if([stats intervalStartDate]) {
         NSPredicate *predicate =
            [NSPredicate predicateWithFormat:@"when >= %@", [stats intervalStartDate]];
             [request setPredicate:predicate];
    }*/    

    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Reward"
                                              inManagedObjectContext:context];
    [request setEntity:entity];

    NSArray *results = [context executeFetchRequest:request error:nil];

    if(results && results.count > 0) {
        NSDictionary *resultsDictionary = [results objectAtIndex:0];
        NSNumber *resultValue = [resultsDictionary objectForKey:@"result"];
        return [resultValue intValue];
    } else {
        return 0;
    }    
}

Strangely enough when I stop the app on the simulator and run it again. I can see all points as I expect.

Answer 1

From

reward.action = [NSNumber numberWithInt:action];

I assume the “action” attribute is stored as NSNumber. You should use the same data type in the corresponding predicate:

NSPredicate *actionPredicate = [NSPredicate predicateWithFormat:@"action = %@",
                                          [NSNumber numberWithInt:action]];

ADDED: To your question about “NSManagedObjectContextDidSaveNotification” vs “NSManagedObjectContextObjectsDidChangeNotification”:

A fetch request with NSDictionaryResultType only fetches the current state in the persistent store , and does not take into account any pending changes, insertions, or deletions in the context.

You can find this information e.g. in the documentation of setIncludesPendingChanges:.

This explains why your fetch request sees only changes that have been saved.