从关系中获取请求CoreData

Question

This is the situation, I have two entities in CoreData.

• Question (Chapter:int, Answers:array)
• Answer (Date, Correct:bool)

And this is how they are related:

• The question entity has the chapter where it belongs and a history of answers to that question (that's a relationship).
• The answer entity knows when the question was answered and wether it was correct or not.
• One question has many answers.

The final result I want is something like this...

Fetch questions WHERE chapter = <some> and (LAST) answer.correct = YES

I'm kind of new with core data and right now I'm doing it with many steps but I wanted to know if I can achieve this with only one NSFetchedResultsController without filtering arrays.

Thank you very much for your help!

Answer 1

Try this predicate. May work for you.

  NSPredicate *predicate = [NSPredicate predicateWithFormat:@"chapter = '%@' AND answer[LAST].correct=TRUE",chapterName];

Answer 2

A little convoluted, but I think the following will get what you want:

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"chapter like %@ AND (SUBQUERY(answer, $t, $t.correct == YES).date.@max = SUBQUERY(answer, $t, $t.correct IN {YES, NO}).date.@max)"];

This checks whether the maximum date for correct answers is equal to the maximum date for all answers.