I have two NSManagedObjects. Both are based off of a selection inside a UITableView.
NSManagedObjects *One = [rootViewController.fetchedResultsController objectAtIndexPath:currentSelection];
NSManagedObjects *Two = [fetchedResultsController objectAtIndexPath:indexPath];
I want to link the two together (to-many), but since the entries already exist, I do not want to use insertNewObjectForEntityForName. If I use insertNewObjectForEntityForName my code works but it links the new entry to an existing entry. I need to link two already existing entries together.
Is this possible and if so, how would you go about it?
Solution found. One must use the same NSManagedObjectContext for both NSManagedObjects. Once I had done so, code worked as expected.
Related
Im currently having a problem with my CoreData implementation. I try to query the number of objects with certain properties. I therefore execute the following code
NSUInteger itemsCount = [managedObjectContext countForFetchRequest:fetchRequest error:NULL];
NSLog(#"%i",(int)itemsCount);
So far so good. Everything works if i work with a fresh MOC where no data has ever been saved. The problem occures when i do the following:
I save the whole MOC
Delete one of those objects matching the fetch criteria from the MOC (without saving!)
Requery the Number of objects matching the criteria
The number of objects does not change although i deleted one of the objects.
I also tried the following query ...
fetchRequest.resultType = NSCountResultType;
NSArray* array = [managedObjectContext executeFetchRequest:fetchRequest error:&fetchError];
NSLog(#"%i",[[array firstObject] intValue]);
... with the same result.
Then I tried querying the objects directly ...
fetchRequest.resultType = NSManagedObjectResultType;
array = [managedObjectContext executeFetchRequest:fetchRequest error:&fetchError];
NSLog(#"%i",(int)[array count]);
... tada: Here it works. The array does not contain the deleted object.
When I set fetchRequest.includesPendingChanges = NO, i of course get all objects because i did not save yet. The property has no effect ob the first two queries though because they show too many objects in the first place.
So my question is, what am i doing wrong and why can i not obtain the right amount of objects once i saved them to the persistence store. Do i have to reset the MOC after saving?
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Update
I searched further for similar problems and found some interesting posts. Fetching objects from the MOC means also querying the persistence store. Since I did not save my changes immediately, I do or don't fetch deleted and newly inserted objects respectively. But strangely this only applies for queries involving dictionary result types or count fetches. When querying the objects itself i retrieve the correct number of objects including newly inserted and without the deleted ones.
Now I am wondering on how to deal with my problem. I have a table view which smoothly works with a NSFetchedResultsController (NSFRC). I do not have to save any changes since the controller handles all the updates and notifies me. But as soon as i insert or delete data and the NSFRC notifies me, i want to do further calculations to update my other views. Those calculations include average and number of objects with certain attribute etc.
Right now I update those view as soon as an insert or delete delegate method is being called. But those queries wont return the correct values.
Calling the following before querying has no effect on those queries.
[moc processPendingChanges]
fetch.includePendingChanges = YES
Apparently my approach is lacking something. How do i fetch my needed values correctly ? Since my NSFRC obviously keeps track of deleted and inserted values im curious if it would help to create a second NSFRC just to fetch the averages and so on? Is it even capable of that?
Thanks!
I am trying to implement iCloud sync in my Core Data app. I am not that pro in programming and this is really an advanced topic I learned... I found that Core Data sync Framework "Ensembles" by Drew McCormack. It seems to make iCloud Sync much easier.
I integrated it in my App and syncing does work quite well as long as I add new objects to my Core Data model. But when I delete an object, it creates duplicates. And then duplicates from duplicates. I ended up having the same Entry (object) like 3-4 times...
Why is that? What am I doing wrong? I did some research and my guess is that global identifiers could solve this?
What are global identifiers? My guess is that they help to avoid duplicates!? But how do I set this? I really have no idea, did a lot of research but couldn´t find an answer to that.
Thanks for help!
Update:
Thanks for help! I read the readme and the book, but since i am beginner not everything is clear to me.
I think I understand the use of global identifiers in Ensembles now, but I don´t know if I´m doing it correctly.
If I understand it right, I have to assign an identifier to each object. I can do this by storing it in an attribute. This identifier can be anything as long as it is unique and a NSString?
In my app the user can store different things, let´s say name, text, title, date and so on. The app is based on the Master-Detail-View template in Xcode and uses Core Data. My Core Data model has only a single entity with some attributes, most are strings and a NSDate. No relationships or anything. If the user hits "+" a new object is created and I store the things the user enters in the attributes.
What I did to add global identifiers is to add a new attribute that stores it.
So when a new object is created i do
/// I did find that to use as identifier !?
NSString *taskUniqueStringKey = newManagedObject.objectID.URIRepresentation.absoluteString;
/// and store it in the attribute.
[newManagedObject setValue:taskUniqueStringKey forKey:#"coreDataObjectID"];
Then i use this:
- (NSArray *)persistentStoreEnsemble:(CDEPersistentStoreEnsemble *)ensemble globalIdentifiersForManagedObjects:(NSArray *)objects
{
return [objects valueForKeyPath:#"coreDataObjectID"];;
}
This seems to work for me. But am I doing it right? Is this the right place to assign a global identifier? I have no awakeFromInsert !?
If this is working, I got the next problem. My app is already live and older entries that the user saved before the update will be missing the global identifier. What can I do about that? I thought what I already got and what is unique and the only thing I can think of is an attribute that saves [NSDate date] when the object is created.
I was trying to use this but I failed because Ensembles will only accept NSString and not NSDate!? Can I use this date attribute, is this unique enough and working as gloabl identifier? And if yes, could you please give me code example in how to convert this from date to string?
Syncing with Ensembles works quite good. No duplicates anymore, you can just switch off iCloud and the entries stay and switch it on again and it syncs like it should without loosing locally stored objects or so. Ensembles is really cool! I am seeing some minor strange behaviors like sometimes sync takes long, sometimes it´s really quick and if I edit things in a short time period on two different devices it gets a bit messed up like an object that I just deleted reappears. But I guess that´s normal? If I take some time between using the app on the different devices everything works fine.
Do I understand it right, there is only that one method to call for sync:
- (void)syncWithCompletion:(void(^)(void))completion
{
if (self.ensemble.isMerging) return;
if (!self.ensemble.isLeeched) {
[self.ensemble leechPersistentStoreWithCompletion:^(NSError *error) {
if (error) NSLog(#"Error in leech: %#", error);
if (completion) completion();
}];
}
else {
[self.ensemble mergeWithCompletion:^(NSError *error) {
if (completion) completion();
}];
}
and you just call it if needed? There is nothing else like doing merge without leeching before, or a method like "this is the actual status - save it like it is now" ?
There are different points in the app where you want to sync. On app start and when terminating will be a good point. In my app there are two points where I should sync I guess: when adding an object and save it to Core Data and when I save changes to the object. I could also provide a button like "sync now". Is this a good approach and do I always just call
[self syncWithCompletion:NULL];
Another question that came up. Can I exclude objects from sync with Ensembles? My app loads tutorial entries as objects once on first app start. I don´t want to sync them if that´s possible somehow?
Thanks a lot for your help! If I could help you with anything like localizing in german or so let me know ! ;)
Yes, this is almost certainly due to not setting up global identifiers for your objects, or at least not doing it properly.
When you leech your ensemble, the local persistent store is imported into the sync data. Without global identifiers, Ensembles will assign random ids to your objects, so it can track them across devices.
Duplicates arise when you leech a second device that has the same data. Ensembles has no way to know that the data represents the same logical objects as on the other device, so it again assigns random ids. Effectively, it treats the objects on each device as being completely independent, so that all end up in your data set after syncing.
The solution is global identifiers. By implementing a CDEPersistentStoreEnsemble delegate method, you can provide Ensembles with global ids, which it can use to identify which objects on different devices belong together.
What should you use for global ids? Often, just a UUID, though for singleton like objects you will just want to pick an id.
You can initialize them in awakeFromInsert. You can store the global ids in attributes on your entities. (Note that if you are migrating an existing app, you will want to check with a fetch if the global ids have been generated BEFORE you try to leech the store for syncing.)
More details are in the README on GitHub and in the book at leanpub.
Update
To answer your update questions:
Yes, an identifier just has to be a string, and immutable. It should not change once assigned.
The NSManagedObjectID is not a very good global identifier, in that it will be different on different devices. We really want something that is global across devices.
If you are starting from scratch, using NSUUID is a good approach. Just create a unique id, and store it in the object.
If you have an existing app, and it has been syncing via another mechanism, you need to come up with a way to provide the same global identifiers on each device. One way to do that is mash up the object properties in some way. Usually that will give you a pretty-close-to-unique value, and it will be good enough for the transition.
As an example, you do a quick fetch, and discover that your objects don't yet have global ids. You go through the objects, and set the global ids to a string comprised of creationDate + text. (You could even shorten this by taking a hash, but it probably isn't that important.) After this initial 'migration' to global identifiers, you would just use UUIDs for any newly created objects.
Note that you don't have to use awakeFromInsert. That is simply a convenient place to put it. As long as you assign the global identifier before saving the object you should be fine.
The easiest way to get a string from an NSDate is to call the description method, but another way would be to get a double using timeIntervalSince1970, and turning that into a string. (Be careful with dates as unique identifiers on their own: often objects created together will have the same creation date.)
You are correct about how you should do a sync: you can simply call syncWithCompletion:.
To answer the question about excluding objects: You can't exclude individual objects, mainly because it could become tricky when those objects have relationships to synced objects. You can handle these objects in one of two ways:
Put them in a separate persistent store, and add that store to the same persistent store coordinator.
Sync the objects, but give them global ids manually, so that the objects are treated the same on each device. Eg. You could just give global ids as 'Sample1', 'Sample2', etc.
To integrate Drew's answer, I guess the two steps are the following.
1 Implement CDEPersistentStoreEnsemble delegate method (see README)
- (NSArray *)persistentStoreEnsemble:(CDEPersistentStoreEnsemble *)ensemble
globalIdentifiersForManagedObjects:(NSArray *)objects {
return [objects valueForKeyPath:#"yourUniqueIdentifier"];
}
2 Generate the unique identifier for a NSManagedObject subclass
- (void)awakeFromInsert {
[super awakeFromInsert];
if (!self.yourUniqueIdentifier) {
self.yourUniqueIdentifier = [[NSUUID UUID] UUIDString];
}
}
In awakeFromInsert you can initialize special default property values, like for example an identifier.
The check is necessary, for example, when you have parent-child contexts. Otherwise you are overwriting the identifier previously set. See Why is awakeFromInsert called twice?.
I am using NSFetchedResultsController to section my data into separate sections. The main sortDescriptor I give is this:
NSSortDescriptor *sortDescriptorSectionLetter = [[NSSortDescriptor alloc] initWithKey:#"sectionLetter" ascending:YES selector:#selector(localizedCompare:)];
Now my sectionLetter has a default value of "#". When the tableView is sectioned and presented, it places the '#' section at the top of the table. I want this to show up at the bottom of the list (like the Contacts app does it). But I can't figure out how to accomplish this.
I've also tried different types of characters as the default instead of #, like � and ~, but this doesn't seem to work either. According to the Core Data Programming Guide, I can't pass in a customized comparison function here.
So I'm not sure what my options are here (other than doing the sectioning myself and losing all the FRC delegate goodness to reload my tableviews with animation). Any good ideas?
Notice the remark in the documentation for NSFetchedResultsController:
Subclassing Notes
You create a subclass of this class if you want to customize the creation of sections and index titles. You override sectionIndexTitleForSectionName: if you want the section index title to be something other than the capitalized first letter of the section name. You override sectionIndexTitles if you want the index titles to be something other than the array created by calling sectionIndexTitleForSectionName: on all the known sections.
I've got a few questions I've been trying to answer for myself (by hunting through the documentation) but I have a feeling I'm missing something.
Any hints (and/or pointers to appropriate documentation) would be much appreciated.
I'm building a Core Data document-based application. There are essentially two entities:
There is a single "Comparison" record associated with each document.
There are potentially many "Node" records associated with each document.
My first question is whether I'm thinking about this correctly. Since there is only a single Comparison object for each document, the attributes of the Comparison are essentially attributes of the Document itself. What (if any) is the preferred way of modeling that?
If a Comparison entity is in fact the right way to go, my next question is how and when to actually instantiate the (single) Comparison object. The user should not have to explicitly "add" the Comparison since there's going to be only one of them associated with the Document. Instead, a single Comparison object should be instantiated and inserted into the managedObjectContext. I've got something like this working already, with code in MyDocument.m that looks like this:
(void)windowControllerDidLoadNib:(NSWindowController *)windowController {
[super windowControllerDidLoadNib:windowController];
[NSEntityDescription insertNewObjectForEntityForName:#"Comparison" inManagedObjectContext:managedObjectContext];
}
However -- if the user creates a new document but then never does any work with it -- for example if he immediately clicks the close button -- then he should not be asked to "Save" the document. He should be asked to save his work only if he's actually entered any information. Is there a preferred way to implement this behavior?
I found this thread while struggling with the exact same issue. I have a table of Entity_A working in my document based Core Data app, but I need to figure out how to handle a required single-instance per document of Entity_B.
I've found something that seems to work. There's probably a better way, but this is getting me past this hurdle for now.
When the document's xib is loaded I simply check to see if an Entity_B has been created. if not, I create one and initialize its attributes.
- (void)windowControllerDidLoadNib:(NSWindowController *)aController
{
[super windowControllerDidLoadNib:aController];
//has an Entity_B been created? if not, create one.
NSError *theError = nil;
NSUInteger count = [[self managedObjectContext] countForFetchRequest:[NSFetchRequest fetchRequestWithEntityName:#"Entity_B"] error:&theError];
if( count == 0 )
{
NSManagedObject *newEntity_B = [NSEntityDescription insertNewObjectForEntityForName:#"Entity_B" inManagedObjectContext:[self managedObjectContext]];
[newEntity_B setValue:[NSNumber numberWithBool:YES] forKey:#"boolAttribute"];
[newEntity_B setValue:[NSNumber numberWithInt:2] forKey:#"intAttribute"];
}
}
I didn't insert that code snippet into the original post correctly. Trying again:
-(void)windowControllerDidLoadNib:(NSWindowController *)windowController {
[super windowControllerDidLoadNib:windowController];
[NSEntityDescription insertNewObjectForEntityForName:#"Comparison" inManagedObjectContext:managedObjectContext];
}
Your question about modelling is not very clear, can you please elaborate on what your "Comparison" entity is supposed to do and what sort of attributes you are assigning to it? It would be handy to see your "Document" entity structure so we can provide some useful input.
With regards to your second question, you could check if your NSManagedObject has been updated before deciding on whether to prompt the user to save their document or not:
if ([documentObject isUpdated]) {
...
}
More details in the documentation here http://developer.apple.com/library/mac/#documentation/Cocoa/Reference/CoreDataFramework/Classes/NSManagedObject_Class/Reference/NSManagedObject.html#//apple_ref/occ/cl/NSManagedObject
Cheers,
Rog
There isn't really a "Document" entity, I was simply using that term to refer to the overall document that is saved when the user invokes the Save menu item. Perhaps there is a better way to refer to this concept? NSPersistentDocument?
Backing up a bit... the central idea of the application is to compare two hierarchical directory structures (a visual recursive "diff").
For now the "Comparison" entity has two string attributes, pathA and pathB, which are the names of the two directories to be compared. Each "Node" entity represents the name of a file down in the directory trees that are being compared. The Node entity contains at least one attribute ("relativePath") which is the path relative to the starting point specified in the Comparison.
My first question was simply whether it makes sense for there to be a "Comparison" entity since there is going to be only one of them instantiated (at some point after the user invokes the "New" menu item).
The second question is really at what point should the single "Comparison" object be instantiated and inserted into the managedObjectContext, i.e. what method is most appropriate to make this happen?
Finally if a "Comparison" object is automatically instantiated (at awakeFromNib time, perhaps?) but the user decides not to proceed, and simply clicks the close button, he should not be prompted to save (right?) What would be the appropriate way to accomplish this? The documentObject will appear to have been updated, because an "empty" Comparison object has in fact already been inserted automatically at startup, but the user has not modified it.
Hope that's clear... thanks.
Original question has been answered. Update addresses related question raised in comments.
Original post:
I am using the MOC save method used in Apple's CoreDataBooks. However, I seem to have use for two layers of MOC merging (three MOCs where the 3rd merges with the 2nd and then the 2nd merges with 1st).
First, I have a tableview (ClassList) listing school classes. Selecting a class pushes a 2-row tableview (AddClass). At AddClass, the first row allows the user to edit the class title. Selecting the second row pushes a tableview (ClassRoster) that displays the student roster for that class. Lastly, selecting a student pushes on another 2-row tableview (AddStudent) where the user can edit the student name and username.
I can add and save classes successfully by using the dual MOC merge method (managedObjectContext and addingManagedObjectContext as employed by CoreDataBooks). I will call the "base MOC" in my first view "MOC1" and call the "scratchpad" MOC "MOC2".
MOC2 temporarily stores changes made to a class object. These changes can then either be saved or canceled, sending a -didFinishWithSave:(BOOL) to the delegate. If I save, the changes made in MOC2 are merged with MOC1. That merge is working perfectly.
Handling changes made to student objects is where I'm going wrong. I thought I could employ MOC3 as a scratchpad for changes to student objects which would merge with MOC2 (when I saved a student object). MOC2 could in turn be saved with MOC1 when I saved the class object.
But I have run into errors with saving MOC3 and adding student objects to class objects because they are in different contexts. I can post code, but first I wanted to ask the bigger question: Am I going about this all the wrong way?
UPDATE:
Mr. Zarra recommended using initWithEntity:insertIntoManagedObjectContext: and setting the MOC to nil, thereby creating a temporary object which could later have its MOC set and saved.
Following his advice, I am attempting to incorporate the following code:
NSManagedObjectModel *managedObjectModel - [[managedObjectContext persistentStoreCoordinator] managedObjectModel];
NSEntityDescription *entity = [[managedObjectModel entitiesByName] objectForKey:#"MyClass"];
MyClass *newClass = [[MyClass alloc] initWithEntity:entity insertIntoManagedObjectContext:nil];
I have run into an error, but I'm not sure it is related to this code yet. I will debug and post what I find.
Yes. You do not need to use more than one NSManagedObjectContext. That example is a very poor one. In your case you should be using a single context and that will remove all of your issues.
If you want a temporary entity, create it with a nil NSManagedObjectContext. When you want to save it you call -setManagedObjectContext: and then save that NSManagedObjectContext.
The only time you realistically want to use more than one NSManagedObjectContext is when you are in a multi-threaded situation.