Question

我正在寻找一个谓词来获取Entity类型的所有托管对象，其值在属性sessionId中重复，其中所有组都是＆＃39; （＆＃34; groups＆＃34;，表示sessionId相等的托管对象）内容＆＃39;属性processed中的标志设置为YES。这可以（慢慢地）完成，但我正在为此寻找一个有效的一个班轮。感谢

这是缓慢的方式：

NSFetchRequest *request = [NSEntityDescription entityForName:@"Entity"
                                      inManagedObjectContext:context];
NSArray *all = [context executeFetchRequest:request error:nil];
NSArray *sessionIds = [all valueForKeyPath:@"@distinctUnionOfObjects.sessionId"];
NSMutableArray *objects = [NSMutableArray array];
for (NSString *sessionId in sessionIds) {
    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"sessionId == %@", sessionId];
    NSArray *inSession = [all filteredArrayUsingPredicate:predicate];
    for(id obj in inSession) {
         if(![obj valueForKey:@"processed"]) continue;
    }

    [objects arrayByAddingObjectsFromArray:processed];
}
NSLog(@"%@", objects);

Answer 1

由于布尔值存储为0和1，因此所有行都为processed = YES的组将具有average(processed) = 1。因此，您可以使用NSFetchRequest propertiesToGroupBy和havingPredicate来获取符合条件的sessionId。然后需要第二次获取以获取Entity个sessionId对象中的NSFetchRequest *fetch = [NSFetchRequest fetchRequestWithEntityName:@"Entity"]; fetch.resultType = NSDictionaryResultType; fetch.propertiesToFetch = @[@"sessionId"]; fetch.propertiesToGroupBy = @[@"sessionId"]; fetch.havingPredicate = [NSPredicate predicateWithFormat: @"average:(processed) == 1"]; NSArray *resultsArray = [context executeFetchRequest:fetch error:nil]; NSArray *sessionIdArray = [resultsArray valueForKeyPath:@"sessionId"]; NSFetchRequest *newFetch = [NSFetchRequest fetchRequestWithEntityName:@"Entity"]; newFetch.predicate = [NSPredicate predicateWithFormat:@"name IN %@",sessionIdArray]; NSArray *finalResults = [context executeFetchRequest:newFetch error:nil]; NSLog(@"Final results, %@", finalResults);个对象：

NSFetchRequestExpression

对不起它不是单行。我留给你，以确定它是否比你自己的代码更快。

修改

要在一次抓取中完成所有操作，请使用NSFetchRequest *fetch = [NSFetchRequest fetchRequestWithEntityName:@"Entity"]; fetch.resultType = NSDictionaryResultType; fetch.propertiesToFetch = @[@"sessionId"]; fetch.propertiesToGroupBy = @[@"sessionId"]; fetch.havingPredicate = [NSPredicate predicateWithFormat: @"average:(processed) == 1"]; NSExpression *fetchExpression = [NSFetchRequestExpression expressionForFetch:[NSExpression expressionForConstantValue:fetch] context:[NSExpression expressionForConstantValue:context] countOnly:false]; NSFetchRequest *newFetch = [NSFetchRequest fetchRequestWithEntityName:@"Entity"]; newFetch.predicate = [NSPredicate predicateWithFormat:@"sessionId IN %@",fetchExpression]; NSArray *finalResults = [context executeFetchRequest:newFetch error:nil]; NSLog(@"Final results, %@", finalResults);代替中间数组：

- (void) addImageViewToScrollView { //create ArrayImages NSMutableArray *tempArray = [[NSMutableArray alloc] init]; for (int x = 0; x < 170; x++) { [tempArray addObject:[UIImage imageNamed:[NSString stringWithFormat:@"%d.jpg",x]]]; } self.imagesArray = [NSArray arrayWithArray:tempArray]; //Frame ScrollView in UIView self.scrollView.contentSize = CGSizeMake(self.scrollView.frame.size.width * self.imagesArray.count, self.scrollView.frame.size.height); //scrollView add subview ImageView for (int i = 0; i < self.imagesArray.count; i++) { CGRect frame; frame.origin.x = self.scrollView.frame.size.width * i; frame.origin.y = 0; frame.size = self.scrollView.frame.size; _imageView = [[UIImageView alloc] init]; _imageView.image = [self.imagesArray objectAtIndex:i]; _imageView.frame = frame; [scrollView addSubview:_imageView]; } self.pageControll.currentPage = 0; self.pageControll.numberOfPages = self.imagesArray.count; }

请注意，在我的（通常是微不足道的）测试设置中，这实际上比双获取解决方案运行得慢。

仅供参考，如果您使用SQLDebug构建设置来检查生成的SQL，它看起来像这样：

SELECT 0，t0.Z_PK，t0.Z_OPT，t0.ZSESSIONID，t0.ZURROCESSED FROM ZENTITY t0 WHERE t0.ZSESSIONID IN（SELECT n1_t0.ZSESSIONID FROM ZENTITY n1_t0 GROUP BY n1_t0.ZSESSIONID，avg（n1_t0.ZPROCESSED）= ？）

核心数据 - NSPredicate具有相同列的对象，也符合另一个条件

1 个答案: