Deletions in a many-to-many structure

问题

I just want to check really quickly. Say I have two entities in a data model: Catalog, and Product. They have a many-to-many relationship with each other, and both are required (a Catalog must have at least one Product, and all Products must each belong to at least one Catalog). So if I was to delete a Product, its deletion should be Nullify, of course.

But what should the deletion policy be for Catalog? If a Catalog is deleted, not all of its Products necessarily exclusively belong to it. A Product may belong to more than one Catalog. So I definitely shouldn't use Cascade. However, is Nullify sufficient? What if I end up with dangling Products that don't belong to a Catalog? What does Core Data have built in that would resolve this issue with many-to-many schemas? Do I need to modify my schema?

回答1:

Nullify is sufficient, and many-to-many sounds right. The specific constraint you want (deleting orphans) is not directly enforceable by core data, though, so you get to do a little cleanup yourself.

Specifically, implement willSave in your entity classes, and have each entity test: am I not deleted; and, do I have no associated (products/catalogs)? If so, delete myself. (the not-deleted test is important to avoid an infinite loop of willSaves.)

This postpones the deletion of the orphaned catalogs or products until save time. This is probably not a problem.

回答2:

I've implemented rgeorge's answer, and thought the exact code might be helpful to other people:

- (void)willSave
{
    [super willSave];

    if (self.isDeleted)
        return;

    if (self.products.count == 0)
        [self.managedObjectContext deleteObject:self];
}

回答3:

Swift translation of Andy and JosephH

override func willSave() {
    super.willSave()

    if self.deleted {
      return
    }

    if self.products.count == 0 {
      self.managedObjectContext?.deleteObject(self)
    }
  }

来源：https://stackoverflow.com/questions/5049164/deletions-in-a-many-to-many-structure