Core Data not saving objects persistently

问题

I'm new to Core Data and as such am not sure if I'm making a mistake. I've downloaded some data from a REST API and it successfully saves the JSON response to disk. I'm trying to process the data and save it persistently using Core Data.

NSLog(@"inserted objects: %@", [managedObjectContext insertedObjects]);
    [managedObjectContext performBlockAndWait:^{
        NSError *error = nil;
        if (![managedObjectContext save:&error]) {
            NSLog(@"Unable to save context for class %@", className);
        } else {
            NSLog(@"saved all records!");
        }
    }];

I've successfully processed the JSON and added it to an NSManagedObjectContext. In the first line, it shows that I've successfully attempted to insert 2 objects.

inserted objects: {(
    <User: 0xa259af0> (entity: User; id: 0xa259b70 <x-coredata:///User/t44BB97D0-C4B4-4BA6-BD25-13CEFDAE665F3> ; data: {
    email = "vishnu@vishnuprem.com";
    experience = "2013-07-20";
    "first_name" = Vishnu;
    id = 2;
    "job_title" = Developer;
    "last_name" = Prem;
    location = "";
    "phone_number" = "+6590091516";
    "profile_pic" = "";
    "thumbnail_profile_pic" = "";
    "user_id" = 2;
}),
    <User: 0xa25e460> (entity: User; id: 0xa25e4c0 <x-coredata:///User/t44BB97D0-C4B4-4BA6-BD25-13CEFDAE665F2> ; data: {
    email = "sanchitbareja@gmail.com";
    experience = "2013-07-20";
    "first_name" = Sanchit;
    id = 1;
    "job_title" = Developer;
    "last_name" = Bareja;
    location = "";
    "phone_number" = "+15106127328";
    "profile_pic" = "";
    "thumbnail_profile_pic" = "";
    "user_id" = 1;
})
)}

When I attempted [managedObjectContext save:&error], it does so successfully and print out "saved all records" as expected. However, when I go to my application .sqlite file and check for added objects, I realize that it hasn't added any objects to the db.

On app relaunch, I print out a list of objects that are already in the database and it confirms that I've none saved yet.

Does anyone know what's going on and why I'm not able to save the data persistently even though it looks like I've successfully created the 'User' objects that needs to be saved in the Core Data model.

EDIT:

here is where I create the NSPersistentStoreCoordinator

// Returns the persistent store coordinator for the application.
// If the coordinator doesn't already exist, it is created and the application's store added to it.
- (NSPersistentStoreCoordinator *)persistentStoreCoordinator
{
    if (_persistentStoreCoordinator != nil) {
        return _persistentStoreCoordinator;
    }

    NSURL *storeURL = [[self applicationDocumentsDirectory] URLByAppendingPathComponent:@"RTModel.sqlite"];

    NSError *error = nil;
    NSLog(@"Test 1");
    _persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:[self managedObjectModel]];
    NSLog(@"Test 2");

    if (![_persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:storeURL options:nil error:&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.

         Typical reasons for an error here include:
         * The persistent store is not accessible;
         * The schema for the persistent store is incompatible with current managed object model.
         Check the error message to determine what the actual problem was.


         If the persistent store is not accessible, there is typically something wrong with the file path. Often, a file URL is pointing into the application's resources directory instead of a writeable directory.

         If you encounter schema incompatibility errors during development, you can reduce their frequency by:
         * Simply deleting the existing store:
         [[NSFileManager defaultManager] removeItemAtURL:storeURL error:nil]

         * Performing automatic lightweight migration by passing the following dictionary as the options parameter:
         [NSDictionary dictionaryWithObjectsAndKeys:[NSNumber numberWithBool:YES], NSMigratePersistentStoresAutomaticallyOption, [NSNumber numberWithBool:YES], NSInferMappingModelAutomaticallyOption, nil];

         Lightweight migration will only work for a limited set of schema changes; consult "Core Data Model Versioning and Data Migration Programming Guide" for details.

         */
        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
        abort();
    }

    return _persistentStoreCoordinator;
}

I have 3 contexts.

• masterManagedObjectContext

• backgroundManagedObjectContext

• newManagedObjectContext

master is parent of both background and new. When I query the contexts like this:

    NSError *error = nil;
    NSFetchRequest *request = [[NSFetchRequest alloc] initWithEntityName:@"User"];
    [request setSortDescriptors:[NSArray arrayWithObject:
                                 [NSSortDescriptor sortDescriptorWithKey:@"id" ascending:YES]]];
    [request setReturnsObjectsAsFaults:NO];
    NSArray *testArray = [[[RTCoreDataController sharedInstance] newManagedObjectContext] executeFetchRequest:request error:&error];

    for (User *obj in testArray) {
        NSLog(@"obj.id %@", obj.id);
    }

    NSLog(@"query records: %@",testArray);

master and background both return the correct obj.id in the NSLog as well as gives the output below for @"query records"

   (
    "<User: 0xa3811d0> (entity: User; id: 0xa381230 <x-coredata:///User/t92BCED2D-CD17-49CC-9EBA-DF8F52F06A002> ; data: {\n    email = \"sanchitbareja@gmail.com\";\n    experience = \"2013-07-20\";\n    \"first_name\" = Sanchit;\n    id = 1;\n    \"job_title\" = Developer;\n    \"last_name\" = Bareja;\n    location = \"\";\n    \"phone_number\" = \"+15106127328\";\n    \"profile_pic\" = \"\";\n    \"thumbnail_profile_pic\" = \"\";\n    \"user_id\" = 1;\n})",
    "<User: 0xa382170> (entity: User; id: 0xa3820b0 <x-coredata:///User/t92BCED2D-CD17-49CC-9EBA-DF8F52F06A003> ; data: {\n    email = \"vishnu@vishnuprem.com\";\n    experience = \"2013-07-20\";\n    \"first_name\" = Vishnu;\n    id = 2;\n    \"job_title\" = Developer;\n    \"last_name\" = Prem;\n    location = \"\";\n    \"phone_number\" = \"+6590091516\";\n    \"profile_pic\" = \"\";\n    \"thumbnail_profile_pic\" = \"\";\n    \"user_id\" = 2;\n})"
)

however "new" returns (null) for the obj.id in NSLog and returns the following for @"query records":

(
    "<User: 0xa2b08a0> (entity: User; id: 0x95aebe0 <x-coredata:///User/tBFCC6C5F-7D2C-4AA0-BA96-B806EE360A762> ; data: <fault>)",
    "<User: 0xa2b0910> (entity: User; id: 0xa4b9780 <x-coredata:///User/tBFCC6C5F-7D2C-4AA0-BA96-B806EE360A763> ; data: <fault>)"
)

回答1:

From your code and the comments it seems that you are not saving the master context. Make sure you call

[managedObjectContext save:&error]; 

on all child contexts that save the data, and after that on the master context as well.

回答2:

I just got done banging my head against essentially the same problem. A UITableViewController fetched a subclass of NSManagedObject from the NSManagedObjectContext, checked if an attribute was nil, and if it was downloaded the data, set that attribute, then saved the NSManagedObjectContext. Something like this:

MyManagedObject *mgObject = //get object from NSFetchResultsController
NSManagedObjectContext *mgObContext = mgObject.managedObjectContext;
if (!mgObject.data)
{
    mgObject.data = [NSData dataWithContentsOfURL:urlWithData];
    [mgObContext performBlock ^{
        NSError *saveError = nil;
        BOOL saveResult = [mgObContext save:&saveError];
        if (saveError || !saveResult)
        {
            NSLog(@"Save not successful..");
        }
    }];
}
//do something with myObject.data

The save function was giving a YES boolean return and saveError was remaining nil, but if I quit the app and relaunched, when my Core Data loaded up my NSManagedObject subclasses, the data attribute was nil, and when this UITableViewController came back up, it had to download the data again.

I couldn't really find a solution to this anywhere… reading through the Core Data documentation didn't help. The solution came to me when I considered the difference between the above code and my code that sets the attributes in the NSManagedObject subclass's factory methods, which is basically:

MyManagedObject *mgObject = [NSEntityForDescription insertNewObjectForEntityName:@"MyManagedObject" inManagedContext:mgObContext];
mgObject.attribute1 = some value
mgObject.attribute2 = another value

The only difference is that I'm calling the factory methods from inside a [mgObContext performBlock:].

So the amended code is:

MyManagedObject *mgObject = //get object from NSFetchResultsController
NSManagedObjectContext *mgObContext = mgObject.managedObjectContext;
if (!mgObject.data)
{
    [mgObContext performBlock: ^{
        mgObject.data = [NSData dataWithContentsOfURL:urlWithData];
        NSError *saveError = nil;
        BOOL saveResult = [mgObContext save:&saveError];
        if (saveError || !saveResult)
        {
            NSLog(@"Save not successful..");
        }
    }];
}
//do something with myObject.data

Which, thus far, is working perfectly. So I think anytime you made modifications to NSManagedObjects' attributes, you need to do so on the their NSManagedObjectContext's thread.

回答3:

Figured I would also add some input to people who may have similar issues.

In my experience, attempting to save objects that don't have sufficient fields filled out don't seem to persist when saving, and no errors seem to be thrown when this is the case. Always double check that your fields are being filled in as expected before the save fires.

Another way to look at these types of issues is to flip the problem on its head. Maybe the object did in fact save, but the method in which you're verifying that they have in fact been saved is wrong. Often you might do this by querying CoreData for the record(s) using certain criteria. Double check that your criteria is correct and that it your query is actually returning what you expect.

If it does not return what you expect, it could be due to your own errors, but it could also be that the array storing your results isn't storing them properly. I have run into cases before where I had to rename an NSArray because something about the array name was causing referencing issues, and thus the array could not point to the results I was expecting. Cheers.

回答4:

I know that this is not an answer to what the OP asked, but I wanted to share my experience about the same subject in case it will help someone else.

I had some issues with saving data persistently, anything seemed to help me fix it. The structure was very simple, an Entity with one field and one relationship (to-many). I made some changes to the class generated, NSMutableOrderedSet instead of NSOrderedSet.

I was not doing multi thread, or anything like that, just adding elements to the relationship. After saving, and re-launching the application, data just disappeared (elements added to the relationship).

I ended up discovering that there is a property called updated. After adding the new element to the relationship, I checked if this property changed its value. It didn't. So I had to create another field in the Entity, a Boolean, just to be able to force the entity to be saved after adding elements to this relationship.

entity.addObject(..)
entity.forceUpdate = true // without this line, it won't update
managedContext.save(..)

So I hope it helps anyone with the same problem, as I spent some time thinking that I was not saving it correctly..

回答5:

Add this after you save your data :

NSError *error = nil;
if (![managedObjectContext save:&error]) {
    NSLog(@"Can't Save! %@ %@", error, [error localizedDescription]);
}

回答6:

I'm a beginner with iOS but I have done some example with CoreData to store users info.

First, you need to create your model with your entity (I suppose you have already done). In my example, my entity is called "User".

First, add a property similar to this

NSManagedObjectContext *context;

to your ViewController class.

Second, in your viewDidLoad method, add this two lines:

AppDelegate *appdelegate = [[UIApplication sharedApplication]delegate];
context = [appdelegate managedObjectContext];

And third, store your info:

NSEntityDescription *entitydesc = [NSEntityDescription entityForName:@"User" inManagedObjectContext:context];
NSManagedObject *newUser = [[NSManagedObject alloc]initWithEntity:entitydesc insertIntoManagedObjectContext:context];
[newUser setValue:(NSString *)[dictionary objectForKey:@"name"] forKey:@"name"];
[newUser setValue:(NSString *)[dictionary objectForKey:@"surname"] forKey:@"surname"];
...

NSError *error;
[context save:&error];

(I take my properties from a NSDictionary called dictionary)

To read your info:

AppDelegate *appdelegate = [[UIApplication sharedApplication]delegate];
context = [appdelegate managedObjectContext];
NSEntityDescription *entitydesc = [NSEntityDescription entityForName:@"User" inManagedObjectContext:context];
NSFetchRequest *request = [[NSFetchRequest alloc]init];
[request setEntity:entitydesc];

//NSPredicate *predicate = [NSPredicate predicateWithFormat:@"NULL"];
[request setPredicate:nil];

NSError *error;
NSArray *matchingData = [context executeFetchRequest:request error:&error];
//NSArray *matchingData = [context executeFetchRequest:nil error:&error];

// If the user is not logged in previously
if (matchingData.count <=0 ){
    //self.displaylabel.text = @"No person find";
} else {
// If the user is already logged in
    for (NSManagedObject *obj in matchingData) {
        AppDataModel *appDataModel=[AppDataModel getInstance];
        appDataModel.appUserInfo = [User alloc]; 
        appDataModel.appUserInfo.name = [obj valueForKey:@"name"];
        appDataModel.appUserInfo.surname = [obj valueForKey:@"surname"];
    }
}

回答7:

After hours of debugging, I found that the reason my updates weren't being saved was because in my subclass of NSManagedObject I defined by properties w/ @synthesize instead of @dynamic.

After I change it, it all saved as expected.

Hope that helped someone.

回答8:

If objects are missing where you dont have an inverse relationship, you need to save both the entities before mapping. Check this example, which I created to demonstrate how core data objects go missing and how to workaround, while working with Core data, for the case where you dont have an inverse relationship

来源：https://stackoverflow.com/questions/18284613/core-data-not-saving-objects-persistently