Quickies for Core Data

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General

• Custom Accessor Methods (scalar) [permalink]
  Say there's a float ivar called radius

  - (float) radius
  {
      [self willAccessValueForKey: @"radius"];
      float f = radius;
      [self didAccessvalueForKey: @"radius"];
  
     return (f);
  
  } // radius
  
  
  - (void) setRadius: (float) newRadius
  {
      [self willChangeValueForKey: @"radius"];
      radius = newRadius;
      [self didChangeValueForKey: @"radius"];
  
  } // setRadius
  

• Custom accessor methods (objects) [permalink]
  Wrap the methods with will/did Access/Change ValueForKey:, and also use KVC to set primitive values:

  - (NSString *) name
  {
      [self willAccessValueForKey: @"name"];
      NSString *string = [self primitiveValueForKey: @"name"];
      [self didAccessValueForKey: @"name"];
  } // name
  
  - (void) setName: (NSString *) x
  {
      [self willChangeValueForKey: @"name"];
      [self setPrimitiveValue: x  forKey: @"name"];
      [self didChangeValueForKey: @"name"];
  } // setName
  

• Inserting an object [permalink]
  Inserting a new object into a managed object context is a multi-stage process. This assumes that self has methods to get the managed object context and managed object model (Apple's CoreData Application template does)

  Thanks to Evan Moseman who pointed me at the new easy way to do it:

          NSManagedObjectContext *moc = [self managedObjectContext];
  	NSManagedObject *obj = [NSEntityDescription 
  		insertNewObjectForEntityForName :@"Condition" 
  		inManagedObjectContext: context];
  

  And the older, more manual way if you need some more control over the process:

      NSManagedObjectContext *moc = [self managedObjectContext];
      NSManagedObjectModel *mom = [self managedObjectModel];
  
      NSEntityDescription *entity;
      entity = [[mom entitiesByName] objectForKey: @"Condition"];
  
      NSString *className;
      className = [entity managedObjectClassName];
  
      NSManagedObject *obj;
      obj = [[NSClassFromString(className) alloc]
                initWithEntity: entity
                insertIntoManagedObjectContext: moc];
  
      [obj autorelease];
  
      [obj setValue: @"nook"  forKey: @"name"];
      [obj setValue: [NSNumber numberWithInt: 23]  forKey: @"ranking"];
  

• Manipulating a to-many relationship [permalink]
  Use mutableSetValueForKey: This returns a proxy that mutates the relationship and does KVO notifications. Think of the name as "[NS]MutableSet" "valueForKey" rather than "mutableSetValue" "forKey", because it returns a mutable set that you manipulate

  NSMutableSet *employees;
  employees = [department mutableSetValueForKey: @"employees"];
  [employees addObject: newEmployee];
  [employees removeObject: sackedEmployee];
  

• Manually sorting results in a fetch request [permalink]
  I haven't figured out how to get the Xcode fetched query editor to sort. In the mean time, here's how to do it in code:

      NSSortDescriptor *sorter;
      sorter = [[NSSortDescriptor alloc]
                   initWithKey: @"when"
                   ascending: YES];
  
      [fetchRequest setSortDescriptors:
                 [NSArray arrayWithObject: sortDescriptor]];
  

• Setting computed default value for a managed object [permalink]
  Make a subclass of NSManagedObject and override awakeFromInsert:

  - (void) awakeFromInsert
  {
      [super awakeFromInsert];
  
      NSDate *date = [NSDate date];
  
      [self setValue: date  forKey: @"when"];
  
  } // awakeFromInsert

• Sorting results [permalink]
  Want data to be sorted in tableviews and whatnot? You can set the "sortDescriptors" binding on an array controller. Write a method that returns the sort descriptors you want:

  - (void) setWhenSortDescriptors: (NSArray *) descriptors
  {
  } // setWhenSortDescriptors
  
  
  - (NSArray *) whenSortDescriptors
  {
      NSSortDescriptor *sorter;
      sorter = [[[NSSortDescriptor alloc]
                    initWithKey: @"when"
                    ascending: NO] autorelease];
  
      return ([NSArray arrayWithObject: sorter]);
  
  } // whenSortDescriptors
  

  This is for a 'permanent' sorting, not allowing the user to change the sorting. Also, new/changed objects added to the collection don't appear to be placed in sorted order.

• Storing Images into the data store [permalink]
  So ya want to store an image into the data store. mmalc says

  Don't use the XML store, but instead use the SQLite store

  If an entity has a large data blob as an attribute, you should make a separate entity for that attribute.

  e.g. a Person has a 'photo' attribute. Create a Photo entity with a single attribute (the data) and a relationship back to the person (if it makes sense, usually it does), and a relationship from the Person to the Photo entity.

  This means that the photo data will only be loaded from the persistent store if you actually use it.

  (markd: this also has the nice side effect of letting the db put all the blob data elsewhere, rather than in with the other data, giving you better locality of reference to your Person data without slogging through all the Photo jazz if you don't need it)

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