Syncing XML with Core Data

As you may have seen from other threads I'm learning iPhone development by writting my first app.

The app uses Core Data to store items from an XML file. What I want to be able to do is read that same XML file at a later date and compare if the items are the same against the one's in core data and if not leave the item if the same, remove if it isn't in the new XML or add if it's a new item. For example the mail application (my app isn't a mail one), after the initial load it will download new emails, leave ones that are still there or delete one's I have deleted via a front end (GMail).

So my question is, is it possible? If so are there any good examples people are aware of?

 

Sorry to bring this up again but I've just got around to implementing it.

I'm trying to keep it simple at the moment so thought I would have two NSMutableArrays, one containing the existing data freshly retrieved from Core Data and the other the new data. Then look for the differences in one field and insert the new objects. However using for loops could end up with a lot of compares, just as an example having 50 items in both could require 2500 checks.

It was then I found removeObjectsInArray. I thought great, one line and it's done. But when I ran through the code I started off with 20 objects in each and ended up with 20 objects when I shouldn't have.

So what am I doing wrong, am I going about it in the best way? Advice along with examples would be gratefully received.

 

I'll let you know now that it is an RSS reader so I don't want a complete replacement as I want to maintain a read/not read indicator for a set amount of items. So in my model I have a FeedInfo entity with a relationship to a FeedItem entity. Here though is hopefully the appropriate code that I'm having the problem with:

PHP:

    NSMutableArray *items;
    NSMutableArray *currentItems;

This is where I'm hoping to remove objects

PHP:

[self currentItems];
    if ([currentItems count] > 0) {
        [items removeObjectIdenticalTo:currentItems];
    }

currentItems

PHP:

- (void)currentItems {
    NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"date" ascending:NO];
    NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:&sortDescriptor count:1];
    
    NSMutableArray *sortedItems = [[NSMutableArray alloc] initWithArray:[feed.feeditems allObjects]];
    [sortedItems sortUsingDescriptors:sortDescriptors];
    self.currentItems = sortedItems;
    
    [sortDescriptor release];
    [sortDescriptors release];
    [sortedItems release];
}

items is a list of objects that have been read from the RSS feed and I use http://github.com/mwaterfall/MWFeedParser to handle that.

 

I think I have cracked it also long as someone can confirm what I'm thinking is true.

My code is now this

PHP:

NSManagedObjectContext *context = [feed managedObjectContext];
    
    
    NSFetchRequest * request = [[NSFetchRequest alloc] init];
    
    [request setEntity:[NSEntityDescription entityForName:@"FeedItem" inManagedObjectContext:context]];
    [request setPredicate:[NSPredicate predicateWithFormat:@"title = %@", item.title]];
    NSUInteger count = [context countForFetchRequest:request error:nil];
    [request release];

    if (count == 0) {
               // Insert if new
        }

Say for example I have two feeds and they just happen to have the same title. Would I run into trouble with the above code or will it work as the NSManagedObjectContext is feed (FeedInfo entity) but I have used the FeedItem entity for the NSEntityDescription?

Lastly if the above is OK do I need to be worried about performing multiple fetches or am I better sticking with it as RSS feeds don't include lots of items?

 

Sorry, I did use removeObjectsInArray in the same way but that didn't work so I must of pasted the code without changing it back. However it still didn't work but would I still have to itterate through?

Do you know of any good documentation on NSSet or is Apple's reference docs OK?

If you don't mind I would also like your opinion on the post above

 

The code would have been, as I said the same as before but with removeObjectsInArray but as requested here it is

PHP:

[self currentItems];
    if ([currentItems count] > 0) {
        [items removeObjectsInArray:currentItems];
    } 

Titles is something I just picked, there is no reason why I can't change this during refactoring. However as requested by entities can be seen in the attached.

IF my NSManagedObjectContext is feed which is a FeedItem object will I have problems if two feeds contain the same or will the NSEntityDescription of FeedItem prevent that? Or by saying using the url as a key for that reason?

 

OK, what I thought worked in post #8 is OK for one feed, but after more testing, if two different threads have the same title it won't work as it seems to be check the whole FeedItem entity rather that the items relating to the current FeedInfo.

So, to ask a specific question, does anyone know how I can do this? Days of Googling isn't helping much!

Also please appreciate that I am new to iPhone development so may need pointing in the right direction more than others! Yes I can read the documentation, but if I could work it out from that do you think I would be posting here