Objective c Core Data Saving many to many relationships

Question

I have a set up where two objects are related by a many to many relationship

artist <<---->> events

ie an artist can be at many events. an event can have many artists attending

So I understand that if the relation is one to many say artist <--->> event then I can just create event objects and assign the artist object to the relationship.

However since both ends are many to many how to I save this relation? Currently I've tried to save the event object first, then I get the artist objects into a NSSet and assign the set to the event pointer to the artists nsset.

But when I go back to query the event.artists, i'm getting 0.

So how do you save many to many relationships for core data?

Answer 1

Whenever I have the need of a many to many relationship, I change it to two one-to-many relationships like this:

1. Create a new entity e.g. ArtistEvent
2. Add a one to many relationship from Artist to ArtistEvent Artist <--->> ArtistEvent
3. Add a one to many relationship from Event to ArtistEvent ArtistEvent <<---> Event

Your overall diagram looks like this:

Artist <--->> ArtistEvent <<---> Event

Now, to create a new relationship between an artist and an event, you just create a new ArtistEvent and set its artist property to the artist and its event property to the event.

This adds an extra level of indirection if you want to find (say) details of all events that an artist will attend, but it's worth it for the improved normalisation of the relationships.

Answer 2

The best way to establish the proper relationships for the object graph that your describing is to create an "Event" entity, and add a relationship (for consistency and clarity sake) call it "artists" which will have a "to-many" connection (which you'll set in the data model inspector in Xcode);

Then, create an entity called "Artist", and similarly create a relationship called "events" with a "to-many" relationship.

Now, in the model editor in Xcode, select the "Event" entity and set the connection of its "artists" relationship, under "Destination" to point to the entity "Artist". Do the same for the "Artist" entity and set its "events" relationship to point to "Events". Make sure to set set the inverses as well (i.e. inverse for "artists" set to "events", inverse for "events" set to "artists").

Now, "events" is a set which contains "Artist" entities, and "artists" is a set which contains "Event" entities. However these sets are NOT mutable.

To add an "Artist" entity to the "artists" set, you first create a mutable proxy set like this

NSManagedObject *someEvent = [NSEntityDescription insertNewObjectForEntityForName:@"Event" inManagedObjectContext:context];
NSManagedObject *newArtist = [NSEntityDescription insertNewObjectForEntityForName:@"Artist" inManagedObjectContext:context];

NSMutableSet *artistMutableSet = [Event mutableSetValueForKey:@"artists"];
[artistsMutableSet addObject:newArtist];

To add an "Event" entity to the "events" set, you do the opposite.