Increment Core data attribute - ios

Question

I have the following relationship in my app:
Product ->> ProductOrder <<- Order
I then have two Obj-c categories in order to etablish these relationship:
ProductOrder+Product
+ (ProductOrder *)addProductToOrderWithProduct:(Product *)product inManagedObjectContext:(NSManagedObjectContext *)context {
ProductOrder *orderProduct = nil;
orderProduct = [NSEntityDescription insertNewObjectForEntityForName:#"ProductOrder" inManagedObjectContext:context];
NSManagedObjectID *productID = [product objectID];
orderProduct.qnty = product.qnty;
orderProduct.price = product.price;
[(Product *)[context objectWithID:productID] addOrderedProductsObject:orderProduct];
return orderProduct;
}
And then Order+ProductOrder
+ (Order *)addOrderedProductToOrderWithOrderedProduct:(ProductOrder *)orderedProduct inManagedObjectContext:(NSManagedObjectContext *)context {
Order *order = nil;
order = [NSEntityDescription insertNewObjectForEntityForName:#"Order" inManagedObjectContext:context];
NSManagedObjectID *orderedProductID = [orderedProduct objectID];
[order addOrderProductsObject:(ProductOrder *)[context objectWithID:orderedProductID]];
return order;
}
I add products to these relationship like so:
for (Product *prod in [self.fetchedResultsController fetchedObjects]) {
[[[DataManager sharedInstance] backgroundManagedObjectContext] performBlock:^{
ProductOrder *prodOrder = [ProductOrder addProductToOrderWithProduct:prod inManagedObjectContext:[[DataManager sharedInstance] backgroundManagedObjectContext]];
Order *order = [Order addOrderedProductToOrderWithOrderedProduct:prodOrder inManagedObjectContext:[[DataManager sharedInstance] backgroundManagedObjectContext]];
NSInteger amount = [order.orderNumber integerValue];
amount++;
order.orderNumber = [NSString stringWithFormat:#"Order %#", [NSNumber numberWithInteger:amount]];
[[DataManager sharedInstance] saveBackgroundContext];
[[DataManager sharedInstance] saveMasterContext];
}];
}
I now want to group these products in headers. I have this orderNumber attribute in my Order entity, I want to increment for every group of products added to the relationship. How would I manage to do this?
Visual example:

Answer 1

Your code has a number of problems.
First, is there a particular reason you need different managed object context? If not, eliminate that part. Remember, you can access each managed object's context with
product.managedObjectContext;
Second, you are using ObjectIDs which is really only necessary if you pass objects across contexts. In your method, you extract the ID from product and then call objectWithID to get it back. That does not make any sense at all.
Third, you need a very good reason to have this ProductOrder entity at all. Even if you have not told us, let's assume it is necessary because you want to include, say, different quantities for the products in an order, as well as an ordering number. However, the name you chose is very confusing. Let's call it Item.
Your scheme should now look like this:
Product <---->> Item <<------> Order
You could simply use the Core Data generated methods to add relationships and throw away your categories.
For your table you could fetch the Order entity and inform the datasource as follows:
// number of sections
fetchedObjects.count;
// title for section
Order *order = fetchedObjects[section];
order.name;
// number of rows in section
Order *order = fetchedObjects[section];
order.items.count;
// row data
Order *order = fetchedObjects[section];
NSArray *items = [order.items sortedArrayUsingSortDescriptors:
#[[NSSortDescriptor sortDescriptorWithKey:#"sequenceNumber" ascending:YES]]];
Item *item = items[indexPath.row];
item.product.name;