Core Data - Breaking A Relationship

Question

I have a Patient entity and a List entity. A Patient can belong to several different lists and a list can have several different patients.

Say I have a patient who belongs to 3 lists (A, B, C). I want to remove the patient from lists A & B. I do not want to delete lists A & B themselves though obviously. How do I go about doing this?

Answer 1

While Tim's answer above is technically correct, it seems like quite a bit of code to me.

I would assume that to remove a patient from the list, you already know that list and have a reference to it at the time you want to remove the patient. Therefore, the code can be as simple as:

id myPatient = ...;
id myList = ...;
[[myPatient mutableSetValueForKey:@"lists"] removeObject:myList];

This is of course assuming that your relationships are bi-directional. If they are not then I strongly suggest you make them bi-directional.

Lastly, because this is a many to many relationship, you can execute the above code in either direction.

[[myList mutableSetValueForKey:@"patients"] removeObject:myPatient];

update

Then the code is even simplier:

[myPatient setLists:nil];

That will remove the patient from all lists.