I am learning coreData and I am new it, I have created a one-to-many relationship of boss and employee, (i.e one boss and many employees). So I am showing all the bosses on firstTableView and when the user clicks on the cells, he can view the employees assigned to each boss and also he can add employees to any particular boss. Now I want to reorder the boss cells. So how it should be done?
Edited based on the discussion below
- (void)insertNewObject:(NSString *)fileName
{
Boss *bossName = [NSEntityDescription insertNewObjectForEntityForName:#"Boss" inManagedObjectContext:self.managedObjectContext];
[bossName setName:fileName];
NSManagedObject *lastObject = [self.controller.fetchedObjects lastObject];
float lastObjectDisplayOrder = [[lastObject valueForKey:#"displayOrder"] floatValue];
NSLog(#"%f",lastObjectDisplayOrder);
[bossName setValue:[NSNumber numberWithDouble:lastObjectDisplayOrder + 1.0] forKey:#"displayOrder"];
// Save the context.
NSError *error = nil;
if (![self.managedObjectContext save:&error]) {
// Replace this implementation with code to handle the error appropriately.
// abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
NSLog(#"Unresolved error %#, %#", error, [error userInfo]);
abort();
}
}
[Specific Answer To Updated Question]
It would be either ....
[self.boss insertObject:newEmployeeObject inEmployeesAtIndex:0];
Which is a core-data generated method that is part of your Boss subclass if you choose to create subclasses from your model. Or....
NSMutableOrderedSet *employees = [self.boss mutableOrderedSetValueForKey:#"employees"];
[employees insertObject:newEmployee atIndex:0]
It's not that intuitive I know, you can't just make a mutable copy, you have to get a special proxy object from mutableOrderedSetValueForKey.
[Original General Answer]...
Core-data now has the ability to use "Ordered Relationships" which you can specify in your model. If you do so, relationships in your object model will be represented by a new class NSOrderedSet which is a hybrid of an NSArray and an NSSet. By re-ordering the objects in this relationship object and saving the context you will reorder the objects in the database and they will maintain their new order. This kind of ordered relationship tends to be used when there isn't natural ordering attribute on the object. For instance the order simply represents the users preference for ordering a list in the UI.
If on the other hand you have an attribute on one of your objects that describes the order for a collection then you can use that attribute to order the results of an NSFetchRequest by specifying the Sort Descriptors. The value of the attribute would specify the position the object would be in in the results of the NSFetchRequest.
If you are using Ordered Relationships you would need keep the order of the NSOrderedSet for that relationship and the UITableView in sync. If the change was driven from the UI then you respond to the UITableViewDataSource delegate methods such as - (void)moveRowAtIndex:(NSUInteger)sourceIndex toIndex:(NSUInteger)destinationIndex and use the information provided to move the corresponding object to it's new position in the core-data relationship either by using the proxy object from mutableOrderedSetValueForKey: or the Core-data generated accessors of a generated subclass.
If the change to order were driven from the data side you would use the methods on UITableView such as insertRowsAtIndexPaths:withRowAnimation: and moveRowAtIndexPath:toIndexPath: to sync the rows in the UITableView with the changes you were making in the data.
If you are using NSFetchRequests you have a similar task. In this case you respond to user driven changes in the order by updating the sort attributes on your objects to match the new order that is described by the UITableView through the UITableViewDataSource delegate. Or if the ordering changes are starting at the data side you update the UITableView through it's methods to match the changes you are making to the sort attributes on the data. In this case you will be working with the results from the NSFetchResults as an NSArray, you would also have to keep that object in sync until the next time you ran the NSFetchRequest. You could use the same sort descriptor to sort the array, or create an NSMutableArray and use it's methods to move the data to match the table.
Finally if you are using NSFetchRequest you may like to look at NSFetchedResultsController It's job it is to simplify task of syncing a sorted NSFetchRequest and a UITableView. There is good sample code for this in the documentation. In this case you may find that the ordering of the data will take care of itself. For instance say your table is ordered by "assignment date" (i.e. the date at which an employee was assigned to a boss) then simply creating the objects with the correct information would trigger the correct results in the table.
Please note that ordered relationships do not work with iCloud. However in my opinion iCloud doesn't work anyway so that's not a problem.
Related
I have a simple to do list application. It uses dynamically generated text fields spaced programmatically for the tasks (I didn't use UITableView because of some custom animations and whatnot that I want to use).
In the app the user can create multiple lists (home, work, school, etc.) each with their own tasks.
I want to use Core Data to store the information
Saving the information is straightforward enough but updating my Core Data objects (List and Task) are where I'm getting stuck. Also how, with Core Data, to associate in a specific tasks with a specific list.
let me know if I need to clarify anything.
Your best bet is NSFetchedResultsController. You can use it exactly like in the pattern suggested by the Xcode templates (you can look at it by creating a new project Master-Detail and checking "User Core Data").
You can device your object model (entity Task) with a string attribute for the name as well as a NSNumber boolean attribute for done etc. I also recommend a timestamp and maybe a serial number for ordering it (I find NSOrderedSet unreliable). Your entity List should have a to-many relationship to Task. Pretty straight forward.
List <---->> Task
The only difference is now to find the right object, because you cannot use objectAtIndexPath. You can use the index in fetchedResultsController.fetchedObjects for that. Just make sure your objects are ordered as expected.
I'm not totally clear on your question, however, the task of updating a managed object is straightforward. When you're doing an initial add (similar to an "insert" in SQL) you might use code like this:
NSManagedObject *obj;
obj = [NSEntityDescription insertNewObjectForEntityForName:#"UserData" inManagedObjectContext:context];
[obj setValue:user forKey:#"userName"];
[obj setValue:goalCategory forKey:#"goalCategory"];
[obj setValue:goalDetail forKey:#"goalDetail"];
NSError __autoreleasing error;
[context save:&error];
That's about it for inserting a new item. For updating after you're found the managed object you're working on, you just change the values in the managed object and use [context save:&error]. Here's an example:
UserData *uData = (UserData *) managedObj;
uData.itemName = nameText;
NSError __autoreleasing *error;
[context save:&error];
That's pretty much it.
As to the update, once you have selected the object(s) to be updated, they are contained in
fetchedResultsController.fetchedObjects
which is an NSArray. So, you might do something like this:
UserData *uData = (UserData *) [fetchedResultsController.fetchedObjects objectAtIndex:3];
uData.completed = YES;
NSError __autoreleasing *error;
[context save:&error];
So, this would update the field completed in the UserData entity to be == YES for the object at index 3 in the fetchedObjects array.
I know there are other methods of updating and lots of options but I haven't found any need for them. fetchedObjects is an array containing the items returned by your fetch; to update them, cast each object to the entity (which is defined as a NSManagedObject), make the change then context save..
HTH.
First of all, think is it good idea to use Core Data for your project. If your model is light and simple, maybe it will be better to use plists.
If you choose Core Data, just remember 2 rules:
Each thread owns separate NSManagedObjectContext;
Perform operations with context only in its thread.
And don't worry about optimizations now. Realize any scheme of updating your storage. Make sure it works. And then you should try some other update methods.
Long question---thanks in advance for your time. After saving new managed objects, I am finding them added to a relationship on another object in my core data database---one for which my code calls no setter method and that has no inverse relationship. I have pored over the code and used logs to isolate the occurrence the best I can, but I'm encountering bizarre behavior I cannot explain (or fix).
More specifically:
I have an entity called PendingSyncTracker. It simply has one relationship, objectsToSync. I have not yet added any line in my code to call a setter method on this relationship. It is a to-many relationship. It points to BaseEntity. For the "Inverse" option, I have selected "No Inverse Relationship."
When I load a particular table view, 3 objects are downloaded from a server and then parsed into managed objects and saved. By the time the table view begins loading cells, 2 of those 3 objects will mystifyingly be present in the objectsToSync relationship.
I have used NSLog all over my code to figure out exactly when these objects can first be found as members of the objectsToSync set.
NSSet *objectsToSync = [[[SyncEngine sharedEngine] fetchClassNamed:#"PendingSyncTracker" withPredicates:nil][0] valueForKey:#"objectsPendingSync"];
NSLog(#"PendingSyncTracker objectsToSync set (%lu objects): %#", (unsigned long)[objectsToSync count], objectsToSync);
The answer to when they first appear in the set actually varies depending on where I do/don't place those 2 lines of code!
The objects are never found on the relationship before the managed object context is saved in the course of saving my 3 new core data objects.
If I don't use those 2 lines till I'm back in the Table View Controller that sent the new objects off to the Sync Engine to be stored locally (where the MOC is accessed and saved), then the log will there reveal that 2 objects have been added to the relationship.
If I use those 2 lines immediately after saving the MOC in the Sync Engine, then the logs will indicate (both there and back in the TVC) that only 1 object has been added to the relationship.
If I use those 2 lines immediately before and after saving the MOC (and back in the TVC), then all 3 logs will reveal that the relationship contains an empty set.
I also have those 2 lines at the beginning of cellForRowAtIndexPath. Regardless of prior logs, that log will always indicate that 2 objects have been added to the relationship.
All 3 of the managed objects that are created in the Sync Engine are stored as entity types that are subEntities of BaseEntity (to which the objectsToSync relationship points). The 2 types that get added to the relationship are each defined to have a reciprocal relationship, but with a different object, not PendingSyncTracker (although the different object is a subEntity of BaseEntity!).
So.. what explains these observations? How are these objects getting added to the relationship?
UPDATE:
- (NSArray*) fetchClassNamed:(NSString*)className withPredicates:(id)parameters;
{
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:className inManagedObjectContext:self.managedObjectContext];
[fetchRequest setEntity:entity];
// set predicates
if (!(parameters == nil)) {
[fetchRequest setPredicate:parameters];
}
NSError *error;
NSArray *fetchedResults = [self.managedObjectContext executeFetchRequest:fetchRequest error:&error];
return fetchedResults;
}
First, what does [[[SyncEngine sharedEngine] fetchClassNamed... do? Just a guess but it is doing something with KVC to set the relationship for you.
Also, you should always, always, always have an inverse relationship. Even if you never use it, Core Data does. Not having an inverse can lead to lots of issues, including but not limited to performance problems and potentially data corruption.
Add an inverse relationship and update your question with what -fetchClassNamed... does.
I have a model, where I have an entity called "Category". This Category has a one-to-many relationship called "Products". How do I set up the fetch request to only get the Category entities WITHOUT the products?
If you query against Category, product elements are loaded as faults. This is the default behavior of CoreData.
On the contrary if you use - (void)setRelationshipKeyPathsForPrefetching:(NSArray *)keys, you can load (pre-fetch) products when the request is executed.
From Core Data App
Faulting reduces the amount of memory your application consumes. A
fault is a placeholder object that represents a managed object that
has not yet been fully realized, or a collection object that
represents a relationship:
A managed object fault is an instance of the appropriate class, but
its persistent variables are not yet initialized. A relationship fault
is a subclass of the collection class that represents the
relationship. Faulting allows Core Data to put boundaries on the
object graph. Because a fault is not realized, a managed object fault
consumes less memory, and managed objects related to a fault are not
required to be represented in memory at all.
and
Pre-fetching is in effect a special case of
batch-faulting, performed immediately after another fetch. The idea
behind pre-fetching is the anticipation of future needs. When you
fetch some objects, sometimes you know that soon after you will also
need related objects which may be represented by faults. To avoid the
inefficiency of individual faults firing, you can pre-fetch the
objects at the destination.
Edit
If you need to count the number of products for a specific category, use - (NSUInteger)countForFetchRequest:(NSFetchRequest *)request error:(NSError **)error using a request like the following:
NSFetchRequest* request = // set up a request for Products
[request setPredicate:[NSPredicate predicateWithFormat:#"toCategory == %#", currentCategory]];
// count for fetch request here...
where toCategory is the inverse relationship from Products to Category and currentCategory is the category you have.
Take a look at the docs for NSFetchRequest
You can set 'includeSubentities' and 'returnsObjectsAsFaults' to restrict what data comes back from Core Data with your model (Product)
https://developer.apple.com/library/mac/#documentation/Cocoa/Reference/CoreDataFramework/Classes/NSFetchRequest_Class/NSFetchRequest.html
I'm having difficulty with a one to one relationship. At the highest level, I have a one to many relationship. I'll use the typical manager, employee, example to explain what I'm trying to do. And to take it a step further, I'm trying to add a one to one House relationship to the employe.
I have the employees being added no problem with the addEmployeesToManagereObject method that was created for me when I subclassed NSManagedObject. When I select an Employee in my table view, I set the currentEmployee of type Employee - which is declared in my .h.
Now that I have that current employee I would like to save the Houses entities attributes in relation to the current employee.
The part that I'm really struggling with is setting the managedObjectContext and setting and saving the houses attributes in relation to the currentEmployee.
I've tried several things but here's my last attempt:
NOTE: employeeToHouse is a property of type House that was created for
me when I subclassed NSManagedObject
House *theHouse = [NSEntityDescription
insertNewObjectForEntityForName:#"House"
inManagedObjectContext:self.managedObjectContext];
// This is where I'm lost, trying to set the House
// object through the employeeToHouse relationship
self.currentEmployee.employeeToHouse
How can I access and save the houses attributes to the currentEmployee?
since House is setup as an Entity it can be considered a table within the data store. If that truly is the case, you need to setup a 1 to 1 relationship between Employee and House in your data model.
If you have already done so, then it is as simple as calling. Although I'm not as familiar with one to one relationships with Core Data as I am with to-many. In either case, try one of the following
[self.currentEmployee addHouseObject: theHouse];
or
self.currentEmployee.employeeToHouse=theHouse;
then to the save to the managedObjectContext:
NSError *error=nil;
if (![self.managedObjectContext save:&error]{
NSLog(#"Core Data Save Error: %#", error);
}
Also, I'm not sure about your particular situation, but your self.managedObjectContext should already be the same as the one pointed to by self.currentEmployee.managedObjectContext.
Good luck,
Tim
In my core data object model I have 3 entities with appropriate relationships so that MyObject can have many MyObjectProperties, and each property can have one MyObjectPropertyImage.
Given a myObject I want to fetch all the images.
I try to do it using the following predicate, however I get an empty array:
[NSEntityDescription entityForName:#"MyObjectPropertyImage" inManagedObjectContext:managedObjectContext];
[NSPredicate predicateWithFormat:#"ANY myObjectProperty.myObject == %#", myObject];
Any ideas?
When working with Core Data it's best to think of your entities as just that: entities in an object graph, instead of tables in a database. Therefore, you don't need to fetch entities related to others using a predicate. Instead, navigate the object graph using the relationships defined in the model. To get all the images related to myObject:
// assuming the relationships are called 'myObjectProperties' and 'myObjectPropertyImage', respectively
NSSet *allImages = [myObject.myObjectProperties valueForKey:#"myObjectPropertyImage"];
Note that this may trigger additional trips to the database if your object graph is not loaded in memory for your myObject entity. To avoid that, make sure you set the pre-fetching relationship keypaths in your fetch request for myObject.
I hope this helps...
Since you have a MyObject instance in hand and it has the relationship path of myObjectProperties-->ObjectProperty-->>PropertyImages you just need to traverse the relationships. It's easy to do this with valueForKeyPath:
Thusly:
NSArray *images=[myObjectInstances valueForKeyPath:#"myObjectProperties.propertyImage"];
(Note: I might have your attribute names wrong but you can get the idea.)
As general rule, you never fetch when have an object from the graph available. You fetch to "pick out thread" of objects matching the predicate and then to find all related objects you follow the thread/relationships to the related objects.