How to count in coredata (aggregation)? - iphone

Question

I am learning core data and particularly working on aggregation.
Current what I want to do : count the number of records from the table which is in to-many relationship with inverse relationship on some criteria.
Currently I am doing this :
NSExpression *ex = [NSExpression expressionForFunction:#"count:"
arguments:[NSArray arrayWithObject:[NSExpression expressionForKeyPath:#"ddname"]]];
NSPredicate *pred = [NSPredicate predicateWithFormat:#"ddtype == 'Home'"];
NSExpressionDescription *ed = [[NSExpressionDescription alloc] init];
[ed setName:#"countDDEvents"];
[ed setExpression:ex];
[ed setExpressionResultType:NSInteger16AttributeType];
NSArray *properties = [NSArray arrayWithObject:ed];
NSFetchRequest *request = [[NSFetchRequest alloc] init];
[request setPredicate:pred];
[request setPropertiesToFetch:properties];
[request setResultType:NSDictionaryResultType];
NSEntityDescription *entity = [NSEntityDescription entityForName:#"DDEvent" inManagedObjectContext:[self.currentAccount managedObjectContext]];
[request setEntity:entity];
NSArray *results = [[self.currentAccount managedObjectContext] executeFetchRequest:request error:nil];
NSDictionary *dict = [results objectAtIndex:0];
NSLog(#"Average birthdate for female heroes: %#", [dict objectForKey:#"countDDEvents"]);
Its from jeff lemarche.
EDIT : and I have found my solution as
NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSPredicate *pred = [NSPredicate predicateWithFormat:#"ddtype == 'Home'"];
[request setPredicate:pred];
NSEntityDescription *entity = [NSEntityDescription entityForName:#"DDEvent" inManagedObjectContext:[self.currentAccount managedObjectContext]];
[request setEntity:entity];
NSError *error = nil;
NSUInteger count = [[self.currentAccount managedObjectContext] countForFetchRequest:request error:&error];
It is working nicely .But I want to do more request of such type at a time . So i think this can't be a preferred way of getting the count .
EDIT :
So I think the approach would be the appropriate one ????
So can anyone tell me more efficient an preferred way of doing this .
Thanks .

Answer 1

I had to count about 10 000 entities and it slowed down my interface responsiveness a lot while doing it with countForFetchRequest..
Here is a way of doing it wth NSExpression:
- (NSUInteger) unfilteredFCsCount {
// Just the fetchRequest
NSFetchRequest *fetchRequest = [self unfilteredFCsFetchRequest];
[fetchRequest setResultType: NSDictionaryResultType];
// You can use any attribute of the entity. its important, because you are not counting
// the properties, but actually the entities
NSExpression *keyPathExpression = [NSExpression expressionForKeyPath: #"sortIndex_"]; // Does not really matter
NSExpression *maxExpression = [NSExpression expressionForFunction: #"count:"
arguments: [NSArray arrayWithObject:keyPathExpression]];
NSExpressionDescription *expressionDescription = [[NSExpressionDescription alloc] init];
[expressionDescription setName: #"fcCount"];
[expressionDescription setExpression: maxExpression];
[expressionDescription setExpressionResultType: NSInteger32AttributeType];
[fetchRequest setPropertiesToFetch: [NSArray arrayWithObject:expressionDescription]];
NSUInteger fcCount = 0;
NSError *error = nil;
NSArray *results = nil;
results = [self.managedObjectContext executeFetchRequest: fetchRequest error: &error];
KSLog(KSLogLevelDebug, #"unfilteredFCsCount results: %#", results);
if([results count] > 0) {
NSNumber *count = [[results objectAtIndex: 0] objectForKey: #"fcCount"];
fcCount = [count intValue];
}
return fcCount;
}

Answer 2

Jeff LaMarche is just using this as a simple example. In practice, this need is so common that Key-Value Coding has a built in macro to handle it and other common collection operations.
See: The Key-Value Programming Guide: Set and Array Operators
In this case you would use the #count operator in your predicate.
Of course, hand tuning your own expression gives you fine control over your predicates but the operators handle 80% of such task.