In software engineering, the active record pattern is a design pattern found in software that stores its data in relational databases. It was named by Martin Fowler in his book Patterns of Enterprise Application Architecture. The interface to such an object would include functions such as Insert, Update, and Delete, plus properties that correspond more-or-less directly to the columns in the underlying database table.
Active record is an approach to accessing data in a database. A database table or view is wrapped into a class; thus an object instance is tied to a single row in the table. After creation of an object, a new row is added to the table upon save. Any object loaded gets its information from the database; when an object is updated, the corresponding row in the table is also updated. The wrapper class implements accessor methods or properties for each column in the table or view.
- Wikipedia
Magical Record for Core Data was inspired by the ease of Ruby on Rails’ Active Record fetching. The goals of this code are to:
- Clean up my Core Data related code.
- Allow for clear, simple, one-line fetches.
- Still allow the modification of the
NSFetchRequestwhen request optimizations are needed.
- In your Xcode Project, add all the
.hand.mfiles from the Source folder into your project. - Add CoreData+MagicalRecord.h file to your
.pchfile or yourAppDelegatefile. - Start writing code! There is no step 3!
To get started, first, import the header file CoreData+MagicalRecord.h in your project’s .pch file. This will allow a global include of all the required headers. If you don’t want auto migration, an in-memory store, or a special name for your stack, simply start working! Otherwise, somewhere in your app delegate, in either the -applicationDidFinishLaunching:withOptions: method, or -awakeFromNib, use one of the following setup calls with the MagicalRecord metaclass:
+ (void) setupAutoMigratingDefaultCoreDataStack;
+ (void) setupCoreDataStackWithInMemoryStore;
+ (void) setupCoreDataStackWithStoreNamed:(NSString *)storeName;
+ (void) setupCoreDataStackWithAutoMigratingSqliteStoreNamed:(NSString *)storeName;
Each call instantiates one of each piece of the Core Data stack, and provides getter and setter methods for these instances. These well known instances to Magical Record, and are recognized as “defaults”.
And, before your app exits, you can use the clean up method:
[MagicalRecord cleanUp];
When using Core Data, you will deal with two types of objects the most: NSManagedObject and NSManagedObjectContext. Magical Record gives you a place for a default NSManagedObjectContext for use within your app. This is great for single threaded apps. You can easily get to this default context by calling:
NSManagedObjectContext *context = [NSManagedObjectContext defaultContext];
This context will be used if a find or request method (described below) is not specifying a specific context using the inContext: method overload.
NSManagedObjectContext *myNewContext = [NSManagedObjectContext context];
This will use the same object model and persistent store, but create an entirely new context for use with threads other than the main thread.
[NSManagedObjectContext contextForCurrentThread];
It is highly recommended that the default context is created and set using the main thread
Most methods in Magical Record return an NSArray of results. So, if you have an Entity called Person, related to a Department (as seen in various Apple Core Data documentation), to get all the Person entities from your persistent store:
NSArray *people = [Person findAll];
Or, to have the results sorted by a property:
NSArray *peopleSorted = [Person findAllSortedByProperty:@"LastName" ascending:YES];
Or, to have the results sorted by multiple properties:
NSArray *peopleSorted = [Person findAllSortedByProperty:@"LastName,FirstName" ascending:YES];
If you have a unique way of retrieving a single object from your data store, you can get that object directly:
Person *person = [Person findFirstByAttribute:@"FirstName" withValue:@"Forrest"];
If you want to be more specific with your search, you can send in a predicate:
NSArray *departments = [NSArray arrayWithObjects:dept1, dept2, ..., nil];
NSPredicate *peopleFilter = [NSPredicate predicateWithFormat:@"Department IN %@", departments];
NSArray *people = [Person findAllWithPredicate:peopleFilter];
NSPredicate *peopleFilter = [NSPredicate predicateWithFormat:@"Department IN %@", departments];
NSArray *people = [Person fetchAllWithPredicate:peopleFilter];
For each of these single line calls, a full stack of NSFetchRequest, NSSortDescriptors, and a simple default error handling scheme (i.e., logging to the console) is created.
NSPredicate *peopleFilter = [NSPredicate predicateWithFormat:@"Department IN %@", departments];
NSFetchRequest *peopleRequest = [Person requestAllWithPredicate:peopleFilter];
[peopleRequest setReturnsDistinctResults:NO];
[peopleRequest setReturnPropertiesNamed:[NSArray arrayWithObjects:@"FirstName", @"LastName", nil]];
...
NSArray *people = [Person executeFetchRequest:peopleRequest];
You can also perform a count of entities in your store, that will be performed on the Store
NSNumber *count = [Person numberOfEntities];
Or, if you’re looking for a count of entities based on a predicate or some filter:
NSNumber *count = [Person numberOfEntitiesWithPredicate:...];
There are also counterpart methods which return NSUInteger rather than NSNumbers:
- (NSUInteger) countOfEntities
- (NSUInteger) countOfEntitiesWithContext:(NSManagedObjectContext *)
- (NSUInteger) countOfEntitiesWithPredicate:(NSPredicate *)
- (NSUInteger) countOfEntitiesWithPredicate:(NSPredicate *) inContext:(NSManagedObjectContext *)
NSPredicate *prediate = [NSPredicate predicateWithFormat:@"diaryEntry.date == %@", today];
int totalFat = [[CTFoodDiaryEntry aggregateOperation:@"sum:" onAttribute:@"fatColories" withPredicate:predicate] intValue];
int fattest = [[CTFoodDiaryEntry aggregateOperation:@"max:" onAttribute:@"fatColories" withPredicate:predicate] intValue];
All find, fetch and request methods have an inContext: method parameter
NSManagedObjectContext *someOtherContext = ...;
NSArray *peopleFromAnotherContext = [Person findAllInContext:someOtherContext];
...
Person *personFromContext = [Person findFirstByAttribute:@"lastName" withValue:@"Gump" inContext:someOtherContext];
...
NSUInteger count = [Person numberOfEntitiesWithContext:someOtherContext];
When you need to create a new instance of an Entity, use:
Person *myNewPersonInstance = [Person createEntity];
or, to specify a context:
NSManagedObjectContext *otherContext = ...;
Person *myPerson = [Person createInContext:otherContext];
To delete a single entity:
Person *p = ...;
[p deleteEntity];
or, to specify a context:
NSManagedObjectContext *otherContext = ...;
Person *deleteMe = ...;
[deleteMe deleteInContext:otherContext];
There is no truncate (delete all entities) operation in core data, so one is provided for you with Active Record for Core Data:
[Person truncateAll];
or, with a specific context:
NSManagedObjectContext *otherContext = ...;
[Person truncateAllInContext:otherContext];
Available only on iOS 4.0 and Mac OS X 10.6.
Paraphrasing the Apple documentation on Core Data and Threading, you should always:
- Use a new, dedicated
NSManagedObjectContextinstance for every thread. - Use an instance of your
NSManagedObjectsthat is local for the newNSManagedObjectContext. - Notify other contexts that the background is updated or saved. < 63BD /ul>
The Magical Record library is trying to make these steps more reusable with the following methods:
+ (void) performSaveDataOperationWithBlock:(CoreDataBlock)block;
+ (void) performSaveDataOperationInBackgroundWithBlock:(CoreDataBlock)block;
CoreDataBlock is typedef’d as:
typedef void (^CoreDataBlock)(NSManagedObjectContext *);
All the boilerplate operations that need to be done when saving are done in these methods. To use this method from the main thread:
Person *person = ...;
[MagicalRecord saveDataInBackgroundWithBlock:^(NSManagedObjectContext *localContext){
Person *localPerson = [person inContext:localContext];
localPerson.firstName = @"Chuck";
localPerson.lastName = @"Smith";
}];
In this method, the CoreDataBlock provides you with the proper context in which to perform your operations, you don’t need to worry about setting up the context so that it tells the default context that it’s done, and should update because changes were performed on another thread.
To perform an action after this save block is completed, you can fill in a completion block:
Person *person = ...;
[MRCoreDataAction saveDataInBackgroundWithBlock:^(NSManagedObjectContext *localContext){
Person *localPerson = [person inContext:localContext];
localPerson.firstName = @"Chuck";
localPerson.lastName = @"Smith";
} completion:^{
self.everyoneInTheDepartment = [Person findAll];
}];
This completion block is called on the main thread (queue), so this is also safe for triggering UI updates.
Magical Record has a dedicated GCD queue on which it operates. This means that throughout your app, you only really have 2 queues (somewhat like threads) performing Core Data actions at any one time: one on the main queue, and another on this dedicated GCD queue.
Magical Record can now import data from NSDictionarys into your Core Data store. This feature is currently under development, and is undergoing updates. Feel free to try it out, add tests and send in your feedback.
This Code is released under the MIT License by Magical Panda Software, LLC.