合并托管对象上下文后，托管对象属性变为零

Question

I have a managed object named SpecialItem and call setSubcategory to change the subcategory. When I save the temporary context and merge with the main context, somehow setSubcategory is called passing in nil as the subcategory. This often results in the SpecialItem object being saved with myProperty set to nil. I don't know what is calling setSubcategory. I'm not explicitly calling setSubcategory:nil.

My question is, what is happening and how can I fix this?

This is the managed object implementation:

@interface SpecialItem : GenericItem
@property (nonatomic, strong) Subcategory *subcategory;
@property (nonatomic, strong) MyProperty *myProperty;
@end

@implementation SpecialItem
@dynamic subcategory;
@dynamic myProperty;

- (void)setSubcategory:(Subcategory *)subcategory
{
   [self willChangeValueForKey:@"subcategory"];
   [self willChangeValueForKey:@"myProperty"];

   [self setPrimitiveValue:subcategory forKey:@"subcategory"];
   [self setPrimitiveValue:subcategory.category.myProperty forKey:@"myProperty"];

   [self didChangeValueForKey:@"myProperty"];
   [self didChangeValueForKey:@"subcategory"];
}
// ...

The managed object contexts are setup like so:

self.tempContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];
self.tempContext.parentContext = self.dataManager.mainContext;

Eventually I have this:

[self saveTempContext];

Here is the saveContext implementation:

- (void)saveContext
{
   LogAndPrint(@"Before save.");
   [self.tempContext performBlockAndWait:^{
      NSError *error = nil;
      if (![self.tempContext save:&error])
      {
         LogAndPrint(@"Error occurred while saving context: %@", error);
      }
   }];

   LogAndPrint(@"Middle of save.");

   [self.dataManager.mainContext performBlockAndWait:^{
      NSError *error = nil;
      if (![self.dataManager.mainContext save:&error])
      {
         LogAndPrint(@"Error occurred while saving context: %@", error);
      }
   }];

   [self.dataManager synchronize];
   LogAndPrint(@"After save.");
}

Here is the synchronize implementation:

- (void)synchronize
{
   LogAndPrint(@"Synchronizing Core Data changes.");
   if (self.persistentContext.hasChanges) {
      [self.persistentContext performBlockAndWait:^{
         NSError *error =  nil;
         if (![self.persistentContext save:&error]) {
            LogAndPrint(@"Error occurred while saving persistent context: %@", error);
         }
      }];
   }
}

Answer 1

I was never able to figure out why this was happening. But I did find a solution that works. I changed the code that was calling setSubcategory to call a new method named [SpecialItem updateSubcategory:subcategory].

- (void)updateSubcategory:(Subcategory *)subcategory
{
   self.subcategory = subcategory;
   self.myProperty = subcategory.category.myProperty;
}

This fixed it and the code has been working fine for several months now.