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copied!<p>Don't do this; what you're doing is porting a pattern from one context to another where it doesn't really make sense.</p> <p>For one thing, you shouldn't be modeling IDs at all in Core Data; the framework does that for you with NSManagedObjectID already. Thus a <code>-clientWithID:</code> method on a CoreDataDAL class is redundant. (Note that I've also changed the name of your hypothetical method to follow proper Cocoa naming conventions.) Instead, you can just use <code>-[NSManagedObjectContext objectWithID:]</code> or <code>-[NSManagedObjectContext existingObjectWithID:error:]</code> to get an object based on its NSManagedObjectID.</p> <p>Similarly, relationship management is handled for you. You don't need to have a method in your DAL that can (say) fetch all of the Address instances that apply for a given Client by evaluating some query. You can just traverse your Client's to-many <code>addresses</code> relationship to get at them, and manipulate the same relationship directly (rather than setting foreign keys etc.).</p> <p>Finally, if you really do want to have methods to perform specialized queries, you can either specify the query via a fetched property on the appropriate entity for its results, or you can add that method <em>directly to the appropriate class</em>. Class methods in Objective-C aren't like static methods in C++, Java or C# - they can be overridden just as instance methods can, and are much more appropriate for this kind of use.</p> <p>For example, say your Client entity has a <code>syncID</code> property representing the ID of the object that it represents in some web service. (Note that this is specifically for relating a local object to a remote object, not the "primary key" of the local object.) You'd probably have class methods on the MyClient class associated with your Client entity like this:</p> <pre><code>@implementation MyClient + (NSString *)entityClassName { return @"Client"; } + (NSEntityDescription *)entityInManagedObjectContext:(NSManagedObjectContext *)context { return [NSEntityDescription entityForName:[self entityClassName] inManagedObjectContext:context]; } + (MyClient *)clientWithSyncID:(NSString *)syncID inManagedObjectContext:(NSManagedObjectContext *)context error:(NSError **)error { MyClient *result = nil; NSFetchRequest *request = [[NSFetchRequest alloc] init]; [request setEntity:[self entityInManagedObjectContext:context]]; [request setPredicate:[NSPredicate predicateWithFormat:@"syncID == %@", syncID]]; [request setFetchLimit:1]; NSArray *results = [context executeFetchRequest:request error:error]; if ([results count] > 0) { result = [results objectAtIndex:0]; } else { if (error != NULL) { *error = [NSError errorWithDomain:MyAppErrorDomain code:MyAppNoClientFoundError userInfo:nil]; } } return result; } @end </code></pre> <p>This is similar to what you wrote in your DAL class, but instead of consolidating all of the fetches in one place, it puts the logic for fetches appropriate to a particular managed object class on that class, which is really where it belongs. Thanks to the fact Objective-C has true class methods, you can actually put methods like <code>+entityInManagedObjectContext:</code> and <code>+entityClassName</code> on a common base class and then override only the latter as appropriate in subclasses (or even have it generate an appropriate entity name from the class name).</p> <p>To sum up:</p> <ul> <li>Don't recreate what Core Data already implements for you in terms of things like object IDs, relationship management, and so on.</li> <li>Leverage polymorphism at both the instance <em>and</em> the class level to keep your code clean, rather than use "utility" classes like "data access layers."</li> </ul>

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