I have two tableviewcontrollers in a navigation controller hierarchy. The first contains parent entities, which I have successfully populated from a core data entity fetch request. I created a one-to-many relationship in the parent entity to the child entity and seem to be adding child entities okay. I can populate it with all the children of all the parents, but can't restrict it to the current parent. I have tried using predicates in the child tableview controller fetch request but without success.
Should I be using includesSubEntities and keeping the parent entity used in the request? If so, would I use the delegate protocol in some way?
The code below is for the child tableview. The commented-out bits are different attempts!
Thanks.
- (NSFetchedResultsController *)fetchSerials
{
if ( !_fetchSerials )
{
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity =
[NSEntityDescription entityForName:#"Serial"
inManagedObjectContext:AppDelegatePtr.managedObjectContext];
// NSPredicate *predicate = [NSPredicate predicateWithFormat:#"inExercise.name = '%#'", [[SMNetworkModel sharedInstance] selectedNarrative]];
// [fetchRequest setPredicate:predicate];
// [fetchRequest setRelationshipKeyPathsForPrefetching:#[#"inExercise"]];
[fetchRequest setEntity:entity];
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:#"narrative" ascending:YES];
[fetchRequest setSortDescriptors:#[sortDescriptor]];
_fetchSerials = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:AppDelegatePtr.managedObjectContext sectionNameKeyPath:nil cacheName:nil];
_fetchSerials.delegate = self;
NSError *error;
if ( ![_fetchSerials performFetch:&error] )
NSLog(#"fetch error: %#", error.localizedDescription);
}
return _fetchSerials;
}
A stupid typo combined with a shortfall in my understanding of my new friend Core Data was all that was stopping Predicates from working.
I had mistakenly thought you passed objectIds around instead of the objects themselves.
Uncomment the two lines involving the predicate and change the selectedNarrative to the correct property and it all worked!
Related
I'm trying to fetch two entities eg: Student with email attribute and Professor, both of have same parent entity eg: Person with attributes entityId, firstName and lastName i want to generate them in two sections using NSFetchedResultsController.
Here is a part from getter for fetchedResultsController
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
[fetchRequest setFetchBatchSize:20];
[fetchRequest setReturnsObjectsAsFaults:NO];
NSEntityDescription *description = [NSEntityDescription entityForName:#"Person"
inManagedObjectContext:self.managedObjectContext];
[fetchRequest setEntity:description];
NSSortDescriptor *firstNameDescriptor = [[NSSortDescriptor alloc] initWithKey:#"firstName" ascending:YES];
NSSortDescriptor *lastNameDescriptor = [[NSSortDescriptor alloc] initWithKey:#"lastName" ascending:YES];
[fetchRequest setSortDescriptors:#[firstNameDescriptor, lastNameDescriptor]];
NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc]
initWithFetchRequest:fetchRequest
managedObjectContext:self.managedObjectContext
sectionNameKeyPath:#"entityId"
cacheName:nil];
All Students have the same entityId and all Professors too
In tableView I have two prototype cells one for Student and another for Professor.
I get two sections as expected but students are in different sections, i have printed all objects from fetchedResultsController in console like this, po [self.fetchedResultsController objectAtIndexPath: [NSIndexPath indexPathForItem:1 inSection:1]] all professors are printed with fault:
<Professor: 0x6080000befc0> (entity: Professor; id: 0xd0000000001c0002 <x-coredata://03A3ECAD-CCA7-424E-86F9-258D25372BA1/Professor/p7> ; data: <fault>)
I have forced the fetch request to return full objects using [request setReturnsObjectsAsFaults:NO] but it had no effect.
Why is it happening so?
To avoid "data fault" issue you should set this field of NSFetchRequest:
request.returnsObjectsAsFaults = NO;
To separate students from professors in two sections you can use multiple NSFetchedResultsControllers, as described here:
https://stackoverflow.com/a/2309855/1689376
To avoid duplication of your code, just create a method like this and call it twice:
- (NSFetchedResultsController) createFetchedResultsController: (NSString *) entityName {
//move your code here
}
Core data is already handling the relationship for me, do I need to build a new query to use the NSFetchedResultsController?
I have "Albums" that contain "Photos".
Album *anAlbum = [...getalbum...];
in the *cellForItemAtIndexPath: I'd like to do something with:
anAlbum.photos
However, I can't convert the indexPath to a NSSet member index. (obviously)
Without Core Data I'd typically just generate the query required myself. I'd like to make use of Core Data (again, obviously).
The "Photo" entity (anAlbum.photos is the relationship) contains the
asset url. I have no issue with the displaying it was more of a
concern of how do I use the NSSet (Core Data relationship) with the
NSFectchedResultsController -- or direct with the view
(collection/table).
First of all I would use a NSFetchedResultsController for this. This component is made to work in conjunction with tables and allows to load data in a lazy loading manner.
Second, the fetch request I would use should be run against Photo entity and not against Album. In other words, you should select all the photos that belong to a specific album.
Here the code, I would use...
- (NSFetchedResultsController *)fetchedResultsController {
if (_fetchedResultsController != nil) {
return _fetchedResultsController;
}
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription
entityForName:#"Photo" inManagedObjectContext:managedObjectContext];
[fetchRequest setEntity:entity];
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"album == %#", anAlbum];
[request setPredicate:predicate];
NSSortDescriptor *sort = [[NSSortDescriptor alloc]
initWithKey:#"takeAt" ascending:NO];
[fetchRequest setSortDescriptors:[NSArray arrayWithObject:sort]];
[fetchRequest setFetchBatchSize:20];
NSFetchedResultsController *theFetchedResultsController =
[[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest
managedObjectContext:managedObjectContext sectionNameKeyPath:nil
cacheName:nil];
self.fetchedResultsController = theFetchedResultsController;
return _fetchedResultsController;
}
So now in your tableView:cellForRowAtIndexPath: you would use
Photo *photo = [_fetchedResultsController objectAtIndexPath:indexPath];
// access the album the photo belongs to
Album* album = photo.album;
You can grasp the main concepts for NSFetchedResultsController at Core Data Tutorial for iOS: How To Use NSFetchedResultsController.
Usually, you're going to convert the set to an NSArray (maybe with NSSet's allobjects) and perhaps sort it or filter it with a NSSortDescriptor or NSPredicate or both. Then, you can access the array by index using the index path section/row.
You can obviously use a NSMutableArray if that's what you need, but it is a pain to try to access NSSet items individually as you essentially have to enumerate them to find what you're looking for. Just create an array. Here's an example where I've done this:
NSArray *tableViewData = [NSArray arrayWithArray:[album.photos allObjects]];
Then, you just access the tableViewData objects with:
UIImage *obj = UIImageJPEGRepresentation([tableViewData objectAtIndex:indexPath.row], 0.50f);
Something like that.
I have two entities, A and B, and the following relationships:
A -> B - To many
B -> A - To one
In other words: A can have zero or more B and B can have only one A.
I want to use NSFetchedResultsController to show my A entries in a table view, but i want to filter the results by A -> B relationship.
To do so, i have a UISegmentedControl, if the user taps the first segment i want to show only the A entries that have at least one relationship with B, and if the second segment is tapped i want to show only the entries with no relationships with B.
I'm using CoreData's NSManagedObject, so my A object has a NSSet property with all B entries in a relationships with A.
This is how i'm instantiating my NSFetchedResultsController:
NSManagedObjectContext *context = self.managedObjectContext;
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:"A" inManagedObjectContext:self.managedObjectContext];
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:descriptorKey ascending:NO];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
[fetchRequest setSortDescriptors:sortDescriptors];
[fetchRequest setEntity:entity];
NSFetchedResultsController *controller = [[NSFetchedResultsController alloc]
initWithFetchRequest:fetchRequest
managedObjectContext:context
sectionNameKeyPath:controllerKey
cacheName:nil];
NSError *error;
BOOL success = [controller performFetch:&error];
if (success) {
return controller;
}
This code get all A entries, how can i make that filter?
You need to add a predicate to your fetch request:
e.g.
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"B.#count == 0"];
[fetchRequest setPredicate:predicate];
This will filter As that don't have any related B objects.
As #Abizern mentioned in comments, you need to add a NSPredicate to your NSFetchedResultsController. The predicate would be something like:
[fetchRequest setPredicate:[NSPredicate predicateWithFormat:#"b == %#", myVarReferenceToB]];
If you only have a unique identifier in B (lets call it identifier) instead of an object reference you could write it as:
[fetchRequest setPredicate:[NSPredicate predicateWithFormat:#"b.identifier == %#", myVarReferenceToBIdentifier]];
This will produce your filter.
Every time the user changes the segmented control you will need to re-build the fetch or you will need to keep one NSFetchedResultsController around per segment.
I'm trying to modify a simple Core Data fetch request for contacts to only look for contacts with a relationship with a certain tag. Contact and Tag are both entities with a many-to-many relationship.
I understand with Core Data I can do this by first fetching the Tag object, and then calling tag.contact, but I don't want to do it this way as the rest of the code is dependent on the fact that the fetchResultsController returns Contact objects, not Tag objects.
If I were to do relational databasing, I could do a simple cross-table query and find all contacts with a certain tag. Is there a simple way I can replicate this via Core Data?
-(NSFetchedResultsController *) fetchedResultsController {
//if fetch controller already exists
if(_fetchedResultsController != nil) {
return _fetchedResultsController;
}
//create a new fetch request
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:#"Contact"
inManagedObjectContext:[self managedObjectContext]];
[fetchRequest setEntity:entity];
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:#"lastName"
ascending:YES];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
[fetchRequest setSortDescriptors:sortDescriptors];
//instantiate the fetch controller with the fetch request and sort by last name into sections
_fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:[self managedObjectContext] sectionNameKeyPath:nil cacheName:nil];
//declare delegate of fetch controller as self
_fetchedResultsController.delegate = self;
NSLog(#"fetchResultsController Created");
return _fetchedResultsController;
}
Use NSPredicate.
Lets say you have related Contacts with Tag by name tags and tag entity has property name.
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"ANY tags.name = [cd] %#", #"sales"];
[fetchRequest setPredicate:predicate];
I am new to iOS Developing, and I'm getting my feet wet in Core Data. I've got an app that I'm slowly piecing together (thanks to help from this site!), but I'm stuck on how to fetch data.
The app:
My app has two main screens, both UITableViews: A "Class List" view where they can add new classes, and an "Add My data model has two entities: Course (for class) and Student, with a to-many relationship from Course-Student. Right now I have it working so that when a I tap on a class in the "Class List" view I am taken to the "Add Students" view where I can add students to the class, but my fetch results controller is returning all students that I have added. My question: how do I format the fetch request in the "Add Students" view to fetch only those students that should belong to that class? Here is the fetched results controller I have right now:
-(NSFetchedResultsController *) fetchedResultsController {
if (_fetchedResultsController !=nil) {
return _fetchedResultsController;
}
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:#"Student" inManagedObjectContext:self.managedObjectContext];
[fetchRequest setEntity:entity];
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:#"name" ascending:YES];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
[fetchRequest setSortDescriptors:sortDescriptors];
_fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:#"name" cacheName:nil];
_fetchedResultsController.delegate = self;
return _fetchedResultsController;
}
I'm guessing it has something to do with predicates, but I haven't gotten that far in my learning. Any help at all would be appreciated. Thanks!
You are guessing right. To fetch all students that belong to a particular course,
add the following predicate to the fetch request:
Course *theCourse = ...; // your Course object
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"course = %#", theCourse];
[fetchRequest setPredicate:predicate];
(assuming that the to-one relationship from Student to Course is called "course").