Core Data: Fetch dictionaries grouped by multiple attributes - cocoa-touch

Question

I have an NSManagedObject subclass Entry with properties type (int) and date (NSDate). Right now I use the following code to get entries for the current date grouped by type.
NSFetchRequest *request = [[NSFetchRequest alloc] initWithEntityName:#"Entry"];
[request setResultType:NSDictionaryResultType];
NSExpression *keyPathExpression = [NSExpression expressionForKeyPath:#"type"];
NSExpression *countExpression = [NSExpression expressionForFunction:#"count:" arguments:#[keyPathExpression]];
NSExpressionDescription *expressionDescription = [[NSExpressionDescription alloc] init];
[expressionDescription setName:#"entryCountByType"];
[expressionDescription setExpression:countExpression];
[expressionDescription setExpressionResultType:NSInteger64AttributeType];
[request setPropertiesToFetch:#[#"type",expressionDescription]];
[request setPropertiesToGroupBy:#[#"type"]];
[request setPredicate:[NSPredicate predicateWithFormat:#"date == %#", [NSDate date]]];
NSError *error;
NSArray *results = [defaultManagedObjectContext() executeFetchRequest:request error:&error];
Results for a day with 1 entry of type 3 and 1 entry of type 5 are:
<_PFArray 0x15e05b50>(
{
entryCountByType = 1;
type = 3;
},
{
entryCountByType = 1;
type = 5;
}
)
I want to fetch without a date predicate at all and have counts for each type listed by date like so (where 1 day has 2 of type 1 and 1 of type 2, and another day has 3 of type 1 and 2 of type 2):
(
{
date = 6/4/14 00:00:00;
type1 = 2;
type2 = 1;
},
{
date = 6/5/14 00:00:00;
type1 = 3;
type2 = 2;
}
)
Is this possible in the way I'm thinking, that is, with a single fetch request? Doing a fetch for each day individually (about 30 sequential fetches) is really slowing down my app. I've tried adding #"date" to propertiesToGroupBy (after removing the date predicate, of course) but all that does is return a similar result to the first output, just with a date param thrown in so that each type for a day is split out into separate dictionaries.

Answer 1

You are confusing "day" and "date". NSDate is accurate down to the millisecond, so you will likely have only unique dates. You cannot group by a date range.
One way is to store the day as a transient property. The scheme does not matter, but one possibility is e.g. year * 10000 + month * 100 + day.
If you want to dispense with the NSExpressions you could just use a method day in your entity that returns a day string (with NSDateFormatter) and use a NSFetchedResultsController with day as the sectionNameKeyPath to display the grouped data.