如何查询财产是否存在关联关系？

Question

我有一个实体Session。在我看来，实体Exercise可以添加到Session中。

在保存之前，我想检查是否与Session有任何关系，如果没有，我将在保存之前删除该对象。

谁能告诉我该怎么做？

编辑：

- (void)createSession
{
    NSCalendar *calendar = [NSCalendar currentCalendar];
    unsigned unitFlags = NSYearCalendarUnit | NSMonthCalendarUnit |  NSDayCalendarUnit;
    NSDateComponents *dateComponentsForToday = [calendar components:unitFlags fromDate:self.picker.date];
    [dateComponentsForToday setHour:0];
    [dateComponentsForToday setMinute:0];
    [dateComponentsForToday setSecond:0];
    NSDate *targetDateBegins = [calendar dateFromComponents:dateComponentsForToday]; 
    NSDate *targetDateEnds = [targetDateBegins dateByAddingTimeInterval:(60 * 60 * 24 - 1)];

    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    [fetchRequest setPredicate:[NSPredicate predicateWithFormat: @"(timeStamp >= %@ && timeStamp <= %@)", targetDateBegins, targetDateEnds]];
    [fetchRequest setEntity:[NSEntityDescription entityForName:@"Session" inManagedObjectContext:managedObjectContext]];
    NSLog(@"timeStamp >= %@ && timeStamp <= %@", targetDateBegins, targetDateEnds);

    NSError *error = nil;
    NSArray *results = [managedObjectContext executeFetchRequest:fetchRequest error:&error];
    NSLog(@"Fetch error: %@", error);

    if ([results count])
    {
        session = (Session *)[results objectAtIndex:0];
    }
    else
    {
        session = (Session *)[NSEntityDescription insertNewObjectForEntityForName:@"Session" inManagedObjectContext:managedObjectContext];
        session.timeStamp = self.picker.date;
    }

    NSSet *filteredExercisesFromSession = [session.exercises filteredSetUsingPredicate:[NSPredicate predicateWithFormat: @"name == %@",selectedExerciseName]];
    if ([filteredExercisesFromSession count] > 0)
    {
        self.exercise = [filteredExercisesFromSession anyObject];
    }

    [fetchRequest release];
    self.fetchedResultsController = nil;
    [setsTableView reloadData];
}

-(IBAction)createSet
{    
    NSSet *filteredExercisesFromSession = [session.exercises filteredSetUsingPredicate:[NSPredicate predicateWithFormat: @"name == %@",selectedExerciseName]];

    if ([filteredExercisesFromSession count] > 0)
    {
        self.exercise = [filteredExercisesFromSession anyObject];
    }
    else
    {
        NSLog(@"exercise does not already exist");
        self.exercise = (Exercise *)[NSEntityDescription insertNewObjectForEntityForName:@"Exercise" inManagedObjectContext:managedObjectContext];
        self.exercise.name = selectedExerciseName;
        self.exercise.muscleGroup = muscleName;
        [session addExercisesObject:exercise];
    }

    Set *set = (Set *)[NSEntityDescription insertNewObjectForEntityForName:@"Set" inManagedObjectContext:managedObjectContext];
    set.weight = [NSNumber numberWithFloat:weightSelected2];
    set.reps = [NSNumber numberWithInt:repSelected];
    set.timeStamp = self.picker.date;
    [self.exercise addSetsObject:set];

    NSError *error = nil;
    if (![managedObjectContext save:&error]) 
    {
        // Handle the error.
    }
    NSLog(@"error: %@", error);
    self.fetchedResultsController = nil; 
    [setsTableView reloadData];
}

Answer 1

编辑多么不清楚的问题......！下次请先贴出代码。

您的 NSPredicate 可能是错误的，请尝试以下操作：

[NSPredicate predicateWithFormat:@"(date >= %@) AND (date <= %@)",
                                   startDate, endDate];

ENDOFEDIT

---

只需获取对象以确保它存在或不存在。

如果您的关系是Session 1->n Exercise，那么您应该有一个反向关系，例如myExercise.session，以便轻松检查（获取）。或者您可以只使用普通的 NSFetchRequest...

另请注意，CoreData 中有“级联”删除功能，但我怀疑它对您的（子实体）情况有帮助。

没有更多的代码/细节，很难弄清楚你的问题是什么以及如何解决它。