I'm trying to edit an entry in a Core Data entry using NSPredicate, but I'm not entirely sure as to how it works.
I'm trying to get an entity based on an id from another object, but I can't see where i'm going wrong. Here's my data model in effect:
Entity: myEntity, Attributes: name, id, value.
I'm trying to retrieve the correct object in the Database via this:
NSEntityDescription *entity = [NSEntityDescription entityForName:#"myEntity" inManagedObjectContext:self.managedObjectContext];
NSFetchRequest *request = [[NSFetchRequest alloc] init];
[request setEntity:entity];
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"id == %#", self.itemToEdit.ID];
[request setPredicate:predicate];
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:#"name" ascending:YES];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
[request setSortDescriptors:sortDescriptors];
NSError *error;
MyEntity *myEntity = [[self.managedObjectContext executeFetchRequest:request error:&error] objectAtIndex: 0];
myEntity.value = self.itemToEdit.anotherValue;
[self.managedObjectContext save:&error];
For whatever reason, the data is not being saved, the app isn't crashing either which leads me to believe the issue is with the predicate. Anyway, what's wrong with the code? As a note, itemToEdit is not of type myEntity, it is another object, but the assignment types are the same.
Regards,
Mike
Case closed guys. Stupid mistake by me. I was so focussed on making this new part of Core Data work (which I find daunting as it is), that I actually forgot to assign self.itemToEdit.value. Sorry for wasting your time.
Mike
Related
In my Core Data model, I have an entity Session and Exercise.
Session has a to many relationship to Exercise (there is a one-one inverse relationship as well).
In my fetch, I am trying to find all Session object that are related to the current Exercise.
I am using the following code which isn't working.
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
[fetchRequest setPredicate:[NSPredicate predicateWithFormat: #"exercise = %#", exercise.name]];
NSEntityDescription *sessionEntity = [NSEntityDescription entityForName:#"Session" inManagedObjectContext:managedObjectContext];
[fetchRequest setEntity:sessionEntity];
NSError *error = nil;
NSArray *results = [managedObjectContext executeFetchRequest:fetchRequest error:&error];
NSLog(#"Fetch error: %#", error);
self.sessionArray = results;
Here is my data model:
First, from the screenshot it seems that your relationship attribute of the Session entity is called exercises not exercise.
Also, it seems to me that it would work if you searched not the Session entity but the Exercise entity and then iterate through the resulting array to extract the sessions.
[fetchRequest setPredicate:[NSPredicate predicateWithFormat:"name == %#", exerciseName]];
NSEntityDescription *exerciseEntity = [NSEntityDescription entityForName:#"Exercise" inManagedObjectContext:managedObjectContext];
[fetchRequest setEntity:exerciseEntity];
...
NSArray *results = [managedObjectContext executeFetchRequest:fetchRequest error:&error];
...
NSMutableArray *mutableSessionArray = [NSMutableArray array];
for (Exercise *ex in results) {
Session *session = [ex exercises];
if (session) [mutableSessionArray addObject:session];
}
self.sessionArray = [NSArray arrayWithArray:mutableSessionArray];
The relationship of the Exercise entity called exercises could be called session for clarity. I would recommend renaming it.
BTW, you can also write = instead of ==, they are equivalent, as far as I know. In the Predicate Programming Guide section on Predicate Format String Syntax it says:
=, ==
The left-hand expression is equal to the right-hand expression.
In your predicate you use =, == is the comparison operator. I think that's where you're going wrong.
Source: http://developer.apple.com/library/mac/#documentation/Cocoa/Reference/Foundation/Classes/NSPredicate_Class/Reference/NSPredicate.html
I have implement a UItable with data sourced via Core Data. The table works fine and presents the data correctly, drills down.. etc.
However, it has the following problem: it presents the content data in a different order every time. I would like it at least to appear consistently or even better alphabetically.
Any ideas on why this might be happening or a specific property or method I should be reviewing in the docuemntation?
Help much appreciated
You need to set an NSSortDescriptor on your NSFetchRequest
NSSortDescriptor *sortDescriptorName = [[NSSortDescriptor alloc] initWithKey:#"name" ascending:YES selector:#selector(caseInsensitiveCompare:)];
[fetchRequest setSortDescriptors:[NSArray arrayWithObject:sortDescriptorName]];
Then create your NSFetchedResultsController with the fetchRequest object.
This would cause your list to appear sorted alphabetically using the property "name" and ignore case while sorting.
You can simply add a sort descriptor to the request. In this example, the data object has a numeric column for "sortOrder", but you could sort on most anything.
- (NSMutableArray *)loadData {
NSManagedObjectContext *context = [self managedObjectContext];
NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:#"NameOfEntity" inManagedObjectContext:context];
[request setEntity:entity];
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:#"sortOrder" ascending:YES];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
[request setSortDescriptors:sortDescriptors];
[sortDescriptors release];
[sortDescriptor release];
NSError *error = nil;
NSMutableArray *mutableFetchResults = [[context executeFetchRequest:request error:&error] mutableCopy];
if (mutableFetchResults == nil) {
// Handle the error.
}
[request release];
return [mutableFetchResults autorelease];
}
I have not done much with CoreData other than to read about it and review some examples, but I would say this:
It sounds like your data is simply coming out of CoreData unsorted, and the order is in fact not guaranteed UNLESS you provide some additional hint about how you would like it sorted, with say a sort descriptor or something.
Sorry I can't provide a concrete reference, but there has to be a way to both fetch data and specify the order in which it is returned.
Alternatively, you could (behind the scenes) fetch all the table data from CoreData and then sort it yourself, but I think that defeats the purpose of using CoreData in the first place and discards a lot of the functionality of CoreData that is likely more efficient than anything you could write yourself to massage the data.
I have a core data entity called images that has just 2 fields:
imageName = NSString
timeStamp = NSNumber
I am trying to simulate a kind of stack LIFO (last in first out).
Inserting a new entry is easy but what about reading the last entry added to the entity?
All images are added with a timestamp, obtained by using
time_t unixTime = (time_t) [[NSDate date] timeIntervalSince1970];
an integer that is equal to the number of seconds since 1970
so, how do I retrieve the last inserted record of a core data (= the record that has the biggest timestamp number)???
thanks
Perform a fetch request, sorting the results by timeStamp.
NSFetchRequest *request = [[[NSFetchRequest alloc] init] autorelease];
[request setEntity:...];
// Results should be in descending order of timeStamp.
NSSortDescriptor *sortDescriptor = [NSSortDescriptor sortDescriptorWithKey:#"timeStamp" ascending:NO];
[request setSortDescriptors:[NSArray arrayWithObject:sortDescriptor]];
NSArray *results = [managedObjectContext executeFetchRequest:request error:NULL];
Entity *latestEntity = [results objectAtIndex:0];
You might also want to restrict the number of results using NSFetchRequest's setFetchLimit:.
I have tried using the method that Chris Doble mentioned and found it to be very slow, especially if there are lot of records that would need to be pulled and checked against the timeStamp. If you want to speed things up, I am now setting an attribute called isMostRecent on my ManagedObject's that I may ever want to get the most recent from. When a new record is to be stored I just grab the most recent record that has this attribute set to YES and change it to NO then set the new record that is being stored to YES. The next time I need to grab to most recent record all I have to do is this...
+ (Photo*)latestPhotoForMOC:(NSManagedObjectContext*)context {
NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:kCoreDataEntityNamePhoto
inManagedObjectContext:context];
[request setEntity:entity];
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"isMostRecent == %#", [NSNumber numberWithBool:YES]];
[request setPredicate:predicate];
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:#"isMostRecent" ascending:NO];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
[request setSortDescriptors:sortDescriptors];
NSError *error = nil;
NSMutableArray *mutableFetchResults = [[context executeFetchRequest:request error:&error] mutableCopy];
Photo* photo = nil;
if (mutableFetchResults && mutableFetchResults.count > 0) {
photo = [mutableFetchResults objectAtIndex:0];
}
return photo;
}
I have found this to be much faster. Yes, it requires a little more on your part to ensure it is used properly and that you don't ever end up with more than one record marked as isMostRecent but for me this was the best option.
Hope this helps someone else too.
In Swift 4, declare:
let context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
let entity = [Entity]()
func getLastRecord() {
let entityCount = (entity.count - 1)
let lastRecord = entity[entityCount] // This is the las attribute of your core data entity
print(lastRecord)
}
I'm trying to use the NSFetchedResultsController in my app, but have a problem to sort my data. I get the following error when trying to sort the result using a relationship that is two levels down from the entity:
* Terminating app due to uncaught exception
'NSInvalidArgumentException', reason:
'to-many key not allowed here'
My data model is set up this way:
Item <<---> Category <--->> SortOrder
<<---> Store
In other words: Each item belongs to one category. Categories can have different sort orders for each store that includes a certain category.
So, I'm creating a fetch request to find all items for a certain store and would like to present the result using category names as sections, and sorted on the sort order.
When I perform the the fetch (last line below), I get the above error.
NSManagedObjectContext* context = [appDelegate managedObjectContext];
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"(status != %d) AND (ANY category.sort.include == YES) AND (ANY category.sort.store == %#)", ItemStatusDefault, store];
NSEntityDescription *entity = [NSEntityDescription entityForName:#"Item" inManagedObjectContext:context];
[fetchRequest setEntity:entity];
[fetchRequest setPredicate:predicate];
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:#"category.sort.order" ascending:YES];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
[fetchRequest setSortDescriptors:sortDescriptors];
[sortDescriptors release];
[sortDescriptor release];
self.resultsController = [[NSFetchedResultsController alloc]
initWithFetchRequest:fetchRequest
managedObjectContext:context
sectionNameKeyPath:#"category.name"
cacheName:nil];
[fetchRequest release];
NSError *error;
BOOL success = [self.resultsController performFetch:&error];
If I change the sorting to, say, category names, it works.
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:#"category.name" ascending:YES];
How can I get the NSSortDescriptor to sort on the sort order?
UPDATE:
So it seems this is not possible. I got a suggestion to create a transient property and sort on that, but Apple documentation clearly states
You cannot fetch using a predicate
based on transient properties
My conclusion here is that I cannot use NSFetchedResultsController out of the box. I need to either access the array of objects the NSFetchResultsController gives me and sort in memory, or setup my own fetch requests and skip NSFetchedResultsController.
iOS 5 provide now ordered relationships
https://developer.apple.com/LIBRARY/ios/releasenotes/DataManagement/RN-CoreData/index.html
UPDATE:
Link updated
Reference : "Core Data Release Notes for OS X v10.7 and iOS 5.0"
I haven't seen any other questions quite like this on here, but I'm hoping someone has some insight. I'm just starting to learn Core Data.
Basically, I have two methods and I want to choose which one to call with an if/else statement based on whether or not the "Contacts" table contains any records. Is there a way using core data to check if there are any records in a table?
The best way I've found so far is to set the fetchLimit to 1 and then check to see if anything returns.
[request setFetchLimit:1];
But I keep thinking there has to be a better/easier way. Anyone know or have a good reference I can look at?
Thanks a ton!
Yes, definitely there is a better method. Setup a fetch request as usual, but, instead of actually executing it, simply ask for the number of objects it would have returned if it had been passed to executeFetchRequest:error:
This can be done using
- (NSUInteger)countForFetchRequest:(NSFetchRequest *)request error:(NSError **)error;
Something like this:
- (int) numberOfContacts{
NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSManagedObjectContext *managedObjectContext = yourManagedObjectContext;
NSEntityDescription *entity = [NSEntityDescription entityForName:#"Contacts" inManagedObjectContext:managedObjectContext];
[request setEntity:entity];
NSError *error = nil;
NSUInteger count = [managedObjectContext countForFetchRequest:request error:&error];
[request release];
if (!error){
return count;
}
else
return -1;
}
It's not necessarily any better or easier, but you can look for a specific record and then create it if it doesn't exist like this:
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:#"Contact"
inManagedObjectContext:[self managedObjectContext]];
[fetchRequest setEntity:entity];
NSError *error;
// Filter based on a predicate
[fetchRequest setPredicate:
[NSPredicate predicateWithFormat:#"identifier == %#", #"1"]];
NSManagedObject *contact = [[managedObjectContext
executeFetchRequest:fetchRequest error:&error] lastObject];
// If the contact was not found
if (!contact)
{
// Create the contact
contact = [NSEntityDescription insertNewObjectForEntityForName:#"Contact"
inManagedObjectContext:managedObjectContext];
[contact setValue:[NSNumber numberWithInt:1] forKey:#"identifier"];
[managedObjectContext save:nil];
}
Marcus Zarra posted some code that demonstrates this in a feed reader app. Marcus is the Core Data master.