NSFetchRequest search for all objects from last Nth date

Question

My data looks like:

objA date1 objnumber2

objB date2 objnumber1

objC date2 objnumber4

objD date2 objnumber3

objE date1 objnumber7

objF date3 objnumber6

objG date1 objnumber5

I am looking for all the objects which are from the Nth last date. The result objects need to be sorted using objnumber (this should be easy using NSSortDescriptor).

So if I specify N=1 (most recent date), I should get [objF] only. (date3 is most recent)

If N=3 (oldest date), I should get sorted [objA, objG, objF]

The sorting part is easy.

My question is do I really need to firstly search for the latest date (using combination of sortdescriptor and nsfetchrequest searchLimit) in the entire data? Then do a second search to find all objects from that date (using nspredicate) and sort it?

Or is there a better way to perform this type of search? How would you generalize this for Nth date instead of last date? That would be a big performance hit no??

NOTE that the date is not known beforehand.

Edit2: okay this is even more complicated since I am using NSDate. So pretty much all the objects have unique dates lol. Gotta throw nsdateformatter in there in the mix too :(

Answer 1

I figured out a solution to my problem.

Instead of using the date, I added a new Int64 NSNumber attribute. Everyday the obj is added, all the objects are tagged with that number for that particular day.

For retriving all objects from Nth day, I firstly do a "fetchlimit 1" nsfetchrequest for that number in a sorted fetchrequest. That gives me the last number.

Now for the Nth last objects, I subtract N from the last number. Then I perform a "nspredicted" nsfetchrequest for all objects with that number attribute. Then I simply just sort the result array.

This resolves my question :D