CoreData change relationships in a loop

Question

I have a simple model like this:

item  category --> category 
      <--- items

and following data:

item A -> category blue
item B -> category blue
item C -> category blue
item D -> category green
item E -> category green

What I want to achieve is to change all items where the category is green to blue.

Following code produced only an error:

Two categories already exist:

blueCategory
greenCategory


for (Item *aItem in [greenCategory.items]) {
    [aItem setCategory:blueCategory];
}

Error:

'NSGenericException', reason: '* Collection <_NSFaultingMutableSet: 0x8a0ef70> was mutated while being enumerated.'

My Question is: How could I alter the category in items within a loop?

Answer 1

The problem is that you can't change (mutate) a collection like your greenCategory.items set while you're looping over it, which is what you're doing when you change the category of the items in that set. What you have to do is create a separate collection and loop over it instead.

NSArray *greenCategoryItemsArray = [greenCategory.items allObjects];

for (Item *aItem in greenCategoryItemsArray) {
    [aItem setCategory:blueCategory];
}