Understanding Core Data delete rules on One to Many
I am a little fuzzy about Core Data Relationships deletion rules. So if someone could help me answer a few questions about them.
I have Entities A and B. A has a to-Many relationship with B, and B has a to-One relationship with A.
A<--->>B
Now, if I set the delete rule at A to Cascade, I understand it will delete all the Bs related to it. But if I set it to Nullify, will it set the Bs to NIL or just the Foreign Key to Nil?
And I looked everywhere about the relationship from B to A, should I set it to Nullify? Will that just Nullate the "B Object" at A? Or will it Nullify all the Bs associated with A? What about Cascade? Will it delete all the Bs associated with A, or just the particular B?
Or do I just use "No Action" on the relation from B to A so that when I delete B, no change will happen to A, but the reference to B won't exist?
I am PRETTY confused with these, so excuse my questions.
Thanks.
Answer
If you set the delete rule to "nullify" and delete the A object, then the references to that object in the Bs will be removed. The inverse works the same way. If you have cascade and delete B then it will remove the A that B pointed to. It will then follow the delete rule from A to the remaining Bs (either cascade or nullify).
The rules you set really depend on your data model. If A were a customer and B were their orders then you could set the A->B rule to deny (prevent A from being deleted if it the customer has orders) or cascade (delete the orders when the customer is deleted). The B->A rule would probably be "nullify". If an order is deleted simply remove the reference to the order from the customer.
The relationship delete rules are described in the Apple Core Data Programming Guide