NSFetchRequest returning nothing

问题

Can anyone spot why this isn't returning any ManagedObjects? I'm trying to add to the ATNSManagedObject+EasyFetching class the following, but the fetch result returns nothing. If I fetch these outside of the EasyFetch class I have 100+ objects so I know it isn't CoreData being empty.

+ (void)deleteAllObjectsInContext;
{
    NSManagedObjectContext *context = [NSManagedObjectContext defaultContext];
    NSEntityDescription *entity = [self entityDescriptionInContext:context];
    NSFetchRequest *request = [[[NSFetchRequest alloc] init] autorelease];
    [request setEntity:entity];
    //[request setIncludesPropertyValues:NO];

    NSError *error = nil;
    NSArray *results = [context executeFetchRequest:request error:&error];
    if (error != nil)
    {
        //handle errors
        NSLog(@"delete error");
    }

    for (NSManagedObject *thing in results) {  <--- |results shows 0 objects|
        [context deleteObject:thing];
    }

    NSError *saveError = nil;
    [context save:&saveError];
}

回答1:

Try simplifying it to:

// first get the context or pass it in as an argument (this is usually what I do for
// a deleteAll class level method like this but your call

+ (void)deleteAllObjectsInContext:(NSManagedObjectContext*)context {

    NSFetchRequest* request = [NSFetchRequest fetchRequestWithEntityName:@ "<yourEntity>"];
    // no predicate
    // no sortDescriptors

    NSError* error = nil;
    NSArray* results = [context executeFetchRequest:request error:&error];

    if (!results || error) { // nil is an error
        // handle error
    }

    // do something with results

}

this way you can avoid having to retrieve an NSEntityDescription object.

UPDATE:

Just wanted to add this passage:

Return Value

"An array of objects that meet the criteria specified by request fetched from the receiver and from the persistent stores associated with the receiver’s persistent store coordinator. If an error occurs, returns nil. If no objects match the criteria specified by request, returns an empty array".

回答2:

One possibility is that your NSManagedObjectContext *context is actually nil. In Objective-C, sending messages to nil is perfectly acceptable, and can make it hard to detect where an issue is.

I don't find documentation for [NSManagedObjectContext defaultContext], so I assume that is a category you wrote (or are using); and I suspect it is not always returning a valid context. Add some logging and see!

回答3:

Try this method and watch the Log. It just fetches everything from the current entity.

+ (NSArray*) retrieveEntity:(NSString*) entityName {

// !!!Here you put your context
NSManagedObjectContext *context = appDelegate.managedObjectContext;

if (context == nil) {
    NSLog(@"Error: No context");
    return nil;
}

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:entityName inManagedObjectContext:context];
[fetchRequest setEntity:entity];

NSError *error = nil;
NSArray *fetchedObjects = [context executeFetchRequest:fetchRequest error:&error];

if (fetchedObjects == nil) {
    // Handle the error
    NSLog(@"Error: No fetched objects.");
    return nil;
}
else 
    NSLog(@"Retrieved objects count:%d", [fetchedObjects count]);

return fetchedObjects;
}

This is an example, how to call it.

    // Retrieve all products
NSArray *flXProducts = [DbConnection retrieveEntity:@"FLXProduct"];

If it returns 0, then there is a problem in your database. You can reveal a problem by finding sql file of your database and tring simple SQL on it in Terminal.

来源：https://stackoverflow.com/questions/11053974/nsfetchrequest-returning-nothing

标签

objective-c

core-data

nsfetchrequest