title: iOS RestKit can not save local entity to database
body: <p>I am using RestKit 0.20 to parse JSON data and save to database.
  THere is a mapped entity SchoolClass, which is handled by RestKit and saves fine.
  I have another entity called MyClass, which stores the classes I have selected. This one is only local on the device. </p>
  
  <p>Here is the code I create and save the MyClass entity</p>
  
  <pre><code> NSManagedObjectContext *managedObjCtx = [RKManagedObjectStore defaultStore].mainQueueManagedObjectContext;
   MyClass* course = [managedObjCtx insertNewObjectForEntityForName:@"MyClass"];
  
   .. set the data for course here
  
   NSError *executeError = nil;
   if(![managedObjCtx save:&amp;executeError]) {
        NSLog(@"Failed to save to data store");
   }
  </code></pre>
  
  <p>Here is the code that initialize the managed data store</p>
  
  <pre><code>  // Initialize managed object store
      NSManagedObjectModel *managedObjectModel = [NSManagedObjectModel mergedModelFromBundles:nil];
      RKManagedObjectStore *managedObjectStore = [[RKManagedObjectStore alloc] initWithManagedObjectModel:managedObjectModel];
      objectManager.managedObjectStore = managedObjectStore;
  
     /**
       Complete Core Data stack initialization
       */
      [managedObjectStore createPersistentStoreCoordinator];
      NSString *storePath = [RKApplicationDataDirectory() stringByAppendingPathComponent:@"RKMainDb.sqlite"];
      NSString *seedPath = [[NSBundle mainBundle] pathForResource:@"RKSeedDatabase" ofType:@"sqlite"];
      NSError *error;
      NSPersistentStore *persistentStore = [managedObjectStore addSQLitePersistentStoreAtPath:storePath fromSeedDatabaseAtPath:seedPath withConfiguration:nil options:nil error:&amp;error];
      NSAssert(persistentStore, @"Failed to add persistent store with error: %@", error);
  
      // Create the managed object contexts
      [managedObjectStore createManagedObjectContexts];
  
      // Configure a managed object cache to ensure we do not create duplicate objects
      managedObjectStore.managedObjectCache = [[RKInMemoryManagedObjectCache alloc] initWithManagedObjectContext:managedObjectStore.persistentStoreManagedObjectContext];
  </code></pre>
  
  <p>It appears the save is successful and in the MyClasseTableViewController I could read the saved MyClass entries. However after I close the app and restart again. The MyClassTableViewController is empty, because the fetched results is empty. I opened the sqlite file using SQLiteBrowser and the MyClass table is empty. Looks like the MyClass entities are only saved in the cache but not in the persistent store. Do I need to call some API provided by RestKit to save it? I tried to read through the doc but could not find it. Please help.</p>