Core Data complex query - core-data

Question

I have model that have field date. I need to fetch all count of records for all day of year or if no record for this day presented fetch as 0. Now i fetch all record for year and make it by hands. Can I do it using core data queries?

Answer 1

Typically in Core Data it is much faster to do all the fetching you will need to do up front rather than breaking it up into multiple fetches.
I tried to count records by day of the year in two ways:
Fetch all the records for a year. Find distinct dates for those records. Loop over the dates and count the records.
Loop over each day in the year. Perform a count-only fetch for the records with that date.
With a small database (48 records), the first approach was about 90 times faster. I imagine the performance of the first approach would get worse as more records are added and the second approach would stay about the same.
Here is the code:
- (void)doFullLoadTestWithYear:(int)year {
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:#"Record" inManagedObjectContext:self.managedObjectContext];
[fetchRequest setEntity:entity];
NSCalendar *gregorian = [[[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar] autorelease];
NSDateComponents *comps = [[[NSDateComponents alloc] init] autorelease];
[comps setDay:1];
[comps setMonth:1];
[comps setYear:year];
NSDate *start = [gregorian dateFromComponents:comps];
[comps setDay:31];
[comps setMonth:12];
NSDate *end = [gregorian dateFromComponents:comps];
NSPredicate *yearPred = [NSPredicate predicateWithFormat:#"date >= %# && date <= %#",start,end];
[fetchRequest setPredicate:yearPred];
NSError *error = nil;
NSArray *array = [self.managedObjectContext executeFetchRequest:fetchRequest error:&error];
[fetchRequest release];
if (error) {
NSLog(#"Error during fetch: %#",[error localizedDescription]);
abort();
}
int dateCount = 0, recordCount = 0;
NSArray *dates = [array valueForKeyPath:#"#distinctUnionOfObjects.date"];
for (NSDate *date in dates) {
NSPredicate *pred = [NSPredicate predicateWithFormat:#"date == %#",date];
NSArray *records = [array filteredArrayUsingPredicate:pred];
dateCount++;
recordCount += [records count];
NSLog(#"%d record(s) with date %#",[records count],date);
}
NSLog(#"Record count for year is %d",recordCount);
NSLog(#"Distinct dates count is %d",dateCount);
}
- (void)doSeparateTestWithYear:(int)year {
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:#"Record" inManagedObjectContext:self.managedObjectContext];
[fetchRequest setEntity:entity];
NSCalendar *gregorian = [[[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar] autorelease];
NSDateComponents *comps = [[[NSDateComponents alloc] init] autorelease];
[comps setDay:1];
[comps setMonth:1];
[comps setYear:year];
NSDate *start = [gregorian dateFromComponents:comps];
[comps setYear:year+1];
NSDate *end = [gregorian dateFromComponents:comps];
NSDateComponents *oneDay = [[[NSDateComponents alloc] init] autorelease];
[oneDay setDay:1];
int dateCount = 0, recordCount = 0;
NSDate *date = start;
while (![date isEqual:end]) {
NSPredicate *dayPred = [NSPredicate predicateWithFormat:#"date == %#",date];
[fetchRequest setPredicate:dayPred];
NSError *error = nil;
int count = [self.managedObjectContext countForFetchRequest:fetchRequest error:&error];
if (error) {
NSLog(#"Error during fetch: %#",[error localizedDescription]);
abort();
}
if (count > 0) {
NSLog(#"%d record(s) with date %#",count,date);
dateCount++;
recordCount += count;
}
date = [gregorian dateByAddingComponents:oneDay toDate:date options:0];
}
NSLog(#"Record count for year is %d",recordCount);
NSLog(#"Distinct dates count is %d",dateCount);
[fetchRequest release];
}