在核心数据中合并三个实体？

Question

I have the following Entities:

Student
  NSString name
  Team inTeam // reverse of Team.students

Team
  NSSet assignments
  NSOrderedSet students // reverse relationship of Student.inTeam

Assignment
  NSDate date // timeless date, only care about the day
  NSNumber assignmentCount
  Team inTeam // reverse of Team.assignmets

No all Students are part of a Team. Not all teams have Assignments. Teams are usually formed by 2 Students (no more than 3).

For all the Teams, I need to display the name of its members and the assignmentCount for a given date.

I can think of different ways to do this, 3 of them:

1. Get all the teams, for each team query Assignment with the given date. This is not efficient (but almost the same as if I were getting everything in a single query having assignmentCount as a fault object?)

2. Keep a collection of all the Teams and keep a collection of all the Assignments with the given date. Only two queries, but need to keep the second collection in sync with the db (the assignmentCount will change, and would rather not do this)

3. Reload the data every time the assignmentCount changes. Obviously not a good solution

Is there a way to get all this information in a single query?

A problem I see is that if I set my predicate to have something like ANY team.assignments.date = %@ it only returns the teams that have assignments.

Any ideas?

Answer 1

Use a fetched results controller (or at least a fetch request that pages in the results). The FRC will manage direct changes on the Team objects and efficient loading from the store when scrolling the list.

You can configure the fetch request to prefetch information from the relationships that you know you will always need. You should use the relationships to get that information, not additional fetch requests.

For assignmentCount for a given date , that is a good candidate for a new fetch. You don't actually need the assignments so use countForFetchRequest:error: and setting a predicate to limit to team == ... .

Answer 2

This is very efficient:

1) Fetch the teams with the appropriate predicate.

[NSPredicate predicateWithFormat:
   @"ANY assignments.date > %@ && assigments.date < %@", 
   dateStartTime, dateEndTime];

2) Access the data like this:

NSOrderedSet *students = team.students;
NSUInteger assignments = team.assignments.count;

That is the reason for Core Data. It will do all the optimizations for you in the background. If you use a NSFetchedResultsController it can notify your table view of changes, all without any effort on your part.

And yes, you are right that the fetch would not include teams without assignments. This would be intended behavior if you want to filter by date.