Core Data objects become null

Question 1

The problem is that (as I wrote in a comment) that the objects are fetched on a background thread, but used on the main (UI) thread. Managed objects can only "live" in the context that they were created in. If the context is deallocated, the objects still exist, but the property accessor methods return just nil.

Possible solutions:

Fetch the objects on the main thread.
Use
```
NSManagedObject *copy = [[mainContext objectWithID:[object objectID]];
```
to "copy" the objects from the background context to the main context. (Perhaps MagicalRecord has a convenience method.)
Instead of fetching managed objects, set
```
[fetchRequest setResultType:NSDictionaryResultType];
[fetchRequest setPropertiesToFetch:@[@"title", ...]];
```
to fetch an array of dictionaries with the attributes you are interested in.

Question 2

Finally found the reason for Null objects. I was calling fetching in background. So Magical Records creates a new Managed Object Context specific for that thread, in NSManagedObject+MagicalFinders.m

+ (NSManagedObjectContext *) MR_contextForCurrentThread;
{
    if ([NSThread isMainThread])
    {
        return [self MR_defaultContext];
    }
    else
    {
        NSMutableDictionary *threadDict = [[NSThread currentThread] threadDictionary];
        NSManagedObjectContext *threadContext = [threadDict objectForKey:kMagicalRecordManagedObjectContextKey];
        if (threadContext == nil)
        {
            threadContext = [self MR_contextWithParent:[NSManagedObjectContext MR_defaultContext]];
            [threadDict setObject:threadContext forKey:kMagicalRecordManagedObjectContextKey];
        }
        return threadContext;
    }
}

So I needed to copy objects from background thread context to main thread context. I finally found the solution here and created my method like this.

+ (void)backgroundFetchWithPredicate:(NSPredicate *)predicate completion:(void(^)(NSArray *, NSError *))completion {
    NSManagedObjectContext *privateContext = [NSManagedObjectContext MR_context];
    [privateContext performBlock:^{
        NSArray *privateObjects = [self MR_findAllWithPredicate:predicate inContext:privateContext];
        NSArray *privateObjectIDs = [privateObjects valueForKey:@"objectID"];
        // Return to our main thread
        dispatch_async(dispatch_get_main_queue(), ^{
            NSPredicate *mainPredicate = [NSPredicate predicateWithFormat:@"self IN %@", privateObjectIDs];
            NSArray *finalResults = [self MR_findAllWithPredicate:mainPredicate];
            completion(finalResults, nil);
        });
    }];
}

Now I just need to call it like this:

[self backgroundFetchWithPredicate:predicate completion:^(NSArray *results, NSError *error) {
            completionHandler(results, error);
        }];

Question 3

I guess it is because you are directly assigning the value to the array. Try assigning value by using mutablecopy

NSArray *tasks = [[self MR_findAllWithPredicate:[NSPredicate predicateWithFormat:@"due_date = nil AND user_id = %@", [DBUsers currentUser].id]] mutableCopy];

Question 4

I've found several weak points in your code, try to work them out:

You never use pageNo parameter in your + (void)getIdeasTasksWithPageNo:(int)pageNo completionHandler:(void (^)(NSArray *, NSError *))completionHandler
You always pass nil for error parameter in the method above
What is the purpose of removing objects from array [self.tasksArray removeAllObjects]; ? (firstly try to comment this line maybe this is the reason)

Question 5

First, check if you are using CoreData in a multi-thread circumstance, if you do, you may need to add some dispatch queue code to make sure your CoreData code running on only one thread. Second, call some other methods of your result array to see if the NSManagedObject is just in a fault status, calling some methods of it will make it fetched.

Question 6

Also, perform one prophylactic check on the implementation of your NSManagedObject class with unexpected Null value in fields. In the .m file of NSManagedObject class, make sure that @dynamic is executed for the fields, and not for example, for some reason, by mistake @synthesize.

// NullFieldsCoreDataObject.h
@interface NullFieldsCoreDataObject : NSManagedObject
@property (nonatomic, retain) NSString * nullField;
@property (nonatomic, retain) NSString * anotherOneNullField;
@property (nonatomic, retain) NSNumber * andAnotherOneNullField;
@end

// NullFieldsCoreDataObject.m
#import "NullFieldsCoreDataObject.h"

@implementation NullFieldsCoreDataObject
// wrong:
//@synthesize nullField, anotherOneNullField, andAnotherOneNullField;
// correctly:
@dynamic nullField, anotherOneNullField, andAnotherOneNullField;
@end