CoreData: many-to-many relationship

问题

I have 2 objects, with many to many relationships.

Here's my code:

+ (NSSet *)activitiesSetForMember:(Members *)member inManagedObjectContext:(NSManagedObjectContext *)context{
NSSet *activitiesSet = nil;

NSFetchRequest *request = [[NSFetchRequest alloc] init];
request.entity = [NSEntityDescription entityForName:@"CompanyActivity" inManagedObjectContext:context];
request.predicate = [NSPredicate predicateWithFormat:@"hasMembers contains %@", member];
NSError *error = nil;
activitiesSet = [[context executeFetchRequest:request error:&error] lastObject];
[request release];

NSLog(@"count: %d", [activitiesSet count]);

//se o array vem vazio (nil), não existe a o tipo de Membro
if (!error && !activitiesSet)
    NSLog(@"Error: Member has no Activities related.");

return activitiesSet;   
}

How can I get all the Activities from a selected Member? How can I construct the request do give me that?

回答1:

As you probably already suspected, something is wrong with your predicate:

Perhaps you are confused because of the way you named your relationships. The names seem to indicate that there is some conceptual muddiness. Let me suggest that you rename your entities and relationships like this:

Members         - Member
CompanyActivity - Activity
memberActivity  - activities
hasMembers      - members

There is a better solution. Does not the attribute activities already contain exactly the activities you want? Thus:

activitiesSet = member.activities; // according to my naming scheme

来源：https://stackoverflow.com/questions/7436164/coredata-many-to-many-relationship