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copied!<p>I had the same conundrum. I like to keep unit test coverage at 100% whenever possible. There is no easy way to generate an organic error condition. In fact, I'm not sure the current implementation of the 4 store types that come with Core Data will <strong>ever</strong> trigger an error in response to executeFetchRequest:error. But as it could happen in the future, here is what I did: </p> <p>I have one unit test case file that is dedicated to validating how my classes handle errors populated by executeFetchRequest:error. I define a subclass of NSIncrementalStore that always produces an error during requests in the implementation file. <code>[NSManagedObjectContext executeFetchRequest:error]</code> is processed by <code>[NSPersistentStoreCoordinator executeRequest:withContext:error:]</code> which processes <code>[NSPersistentStore executeRequest:withContext:error:]</code> on all stores. You may notice that the word "fetch" drops when you move to the coordinator - <em>saves and fetch requests are handled by the same method</em> <code>executeRequest:withContext:error:</code>. So I get coverage for testing against save errors and fetch requests by defining a NSPersistentStore that will always respond to saves and fetches with errors.</p> <pre><code>#define kErrorProneStore @"ErrorProneStore" @interface ErrorProneStore : NSIncrementalStore @end @implementation ErrorProneStore - (BOOL)loadMetadata:(NSError **)error { //Required - Apple's documentation claims you can omit setting this, but I had memory allocation issues without it. NSDictionary * metaData = @{NSStoreTypeKey : kErrorProneStore, NSStoreUUIDKey : @""}; [self setMetadata:metaData]; return YES; } -(void)populateError:(NSError **)error { if (error != NULL) { *error = [[NSError alloc] initWithDomain:NSCocoaErrorDomain code:NSPersistentStoreOperationError userInfo:nil]; } } - (id)executeRequest:(NSPersistentStoreRequest *)request withContext:(NSManagedObjectContext *)context error:(NSError **)error { [self populateError:error]; return nil; } - (NSIncrementalStoreNode *)newValuesForObjectWithID:(NSManagedObjectID *)objectID withContext:(NSManagedObjectContext *)context error:(NSError **)error { [self populateError:error]; return nil; } - (id)newValueForRelationship:(NSRelationshipDescription *)relationship forObjectWithID:(NSManagedObjectID *)objectID withContext:(NSManagedObjectContext *)context error:(NSError **)error { [self populateError:error]; return nil; } - (NSArray *)obtainPermanentIDsForObjects:(NSArray *)array error:(NSError **)error { [self populateError:error]; return nil; } @end </code></pre> <p>Now you can construct the Core Data stack using the ErrorProneStore and be guaranteed your fetch requests will return nil and populate the error parameter. </p> <pre><code>- (void)testFetchRequestErrorHandling { NSManagedObjectModel * model = [NSManagedObjectModel mergedModelFromBundles:nil]; [NSPersistentStoreCoordinator registerStoreClass:[ErrorProneStore class] forStoreType:kErrorProneStore]; NSPersistentStoreCoordinator * coordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:model]; NSManagedObjectContext * context = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType]; [context setPersistentStoreCoordinator:coordinator]; [coordinator addPersistentStoreWithType:kErrorProneStore configuration:nil URL:nil options:nil error:nil]; NSFetchRequest * request = [NSFetchRequest fetchRequestWithEntityName:@"AValidEntity"]; NSError * error; [context executeFetchRequest:request error:&error]; STAssertNotNil(error, @"Error should always be nil"); } </code></pre>

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