问题
I have a Parent entity which has to-many Child. If I order the Parent fetch with a property (e.g. name), everything works fine. But if I try this:
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"child.@count" ascending:NO];
It doesn't work. I've also tried to add
[fetchRequest setRelationshipKeyPathsForPrefetching:[NSArray arrayWithObject:[entityProperties objectForKey:@"child"]]];
But it didn't work as well.
Am I allowed to sort by to-many.@count or do I need to add a property and update it each time I add a child?
回答1:
Assuming you are using a sqlite store; you cannot sort by @count because it is resolved at the "object" level whereas the NSSortDescriptor on the NSFetchRequest is resolved at the database level. Therefore, as you guessed, you would need to create a counting property to sort by.
If you are using a binary or in-memory store type then count should work fine since they are only accessed as objects.
来源：https://stackoverflow.com/questions/7839265/nssortdescriptor-with-to-many-relationship