NSFetchRequest 检索 CoreData 实体中每个科目的每个学生的最高考试分数

Posted 2023-02-27

【中文标题】NSFetchRequest 检索 CoreData 实体中每个科目的每个学生的最高考试分数

【英文标题】：NSFetchRequest to retrieve Every Student's Maximum Test Score for Every Subject in CoreData Entity

【发布时间】：2015-06-17 23:24:24

【问题描述】：

我有一个 CoreData 实体，它在不同长度的比赛中具有多个性能记录。我想在每个比赛中为每个运动员获得最佳表现，然后将结果放在CoreData Table View中。

这是我为实现此目的而想出的技巧。但是，这会干扰我将结果放入 TableView 中，其中比赛距离是部分标题：

- (void)setupFetchedResultsController // attaches an NSFetchRequest to this UITableViewController

MarksFromMeets* results = nil;

NSMutableArray * bestMarks = [[NSMutableArray alloc] init];


NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"MarksFromMeets"];

//    NSLog(@"self.athlete.athleteID = %@",self.athlete.athleteID);

request.predicate = [NSPredicate predicateWithFormat:@"(ANY whoRan.athleteID in %@) AND (eventPR > 0)",
                     self.athleteIDsForPredicate];
/* Call the records for selected athletes (PLURAL) */


request.sortDescriptors = [NSArray arrayWithObjects:
                           [NSSortDescriptor sortDescriptorWithKey:@"whoRan.athleteID" ascending:NO],
                           [NSSortDescriptor sortDescriptorWithKey:@"event" ascending:YES],
                           [NSSortDescriptor sortDescriptorWithKey:@"sortMark" ascending:YES],
                           nil];

以下代码块中的代码是我为每个运动员提取最佳表现的地方。

NSArray * allMarks = [self.headToHeadManagedObjectContext executeFetchRequest:request error:nil];

MarksFromMeets* tempMark;

for (MarksFromMeets* athletePRs in allMarks) 

    if (tempMark == nil) 
        tempMark = athletePRs;
     else 
        if ([athletePRs.whoRan.athleteID isEqualToString:tempMark.whoRan.athleteID]) 
            if ([athletePRs.event isEqualToString:tempMark.event]) 
                if (athletePRs.sortMark < tempMark.sortMark) 
                    tempMark = athletePRs;
                
             else 
                [bestMarks addObject:tempMark];
                tempMark = athletePRs;
            
         else 
            [bestMarks addObject:tempMark];
            tempMark = athletePRs;
        
    

if (debug == 1) NSLog(@"bestMarks count: %lu \nallMarks count: %lu", (unsigned long)[bestMarks count], [allMarks count]);

[self setResultsArray:bestMarks];

NSArray bestMarks 有 27 个 NSManagedObjects，而 fetch 请求返回 35 个。

下面的代码是我过去并且仍然希望填充我的 CoreData TableView 的方式。

self.fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:request
                                                                    managedObjectContext:self.headToHeadManagedObjectContext
                                                                      sectionNameKeyPath:@"event"
                                                                            cacheName:nil];

NSError* error = nil;

results = [[self.headToHeadManagedObjectContext executeFetchRequest: request error:&error]objectAtIndex:0];

NSError* requestError = nil;
NSUInteger myCount = [self.headToHeadManagedObjectContext countForFetchRequest:request error:&requestError];
NSLog(@"In %@ and count of results = %lu", NSStringFromClass([self class]),(unsigned long)myCount);
if (!myCount || myCount == 0) 
    NSMutableString* titleText = [NSMutableString stringWithFormat:@"No Records Found"];
    NSMutableString* messageText = [NSMutableString stringWithFormat:@"There were no records found for the %lu athletes selected. Contact ITrackPerfomance using this list of Athlete ID's: %@",  (unsigned long)[self.athleteIDsForPredicate count], self.athleteIDsForPredicate];

    NSString* cancelText = @"Okay";

    [self displayAlertBoxWithTitle:(NSString*)titleText message:(NSString*) messageText cancelButton:(NSString*) cancelText];

有没有办法用 NSFetchRequest 谓词做到这一点？

或者我可以通过将托管对象放入 NSDictionaries 的 NSArray 中来创建部分，其中每个 NSDictionary 是不同的比赛距离？

感谢您的帮助。如果我完全偏离了正确的道路并且有更有效的方法来实现这一点，我也会很高兴知道这一点！

【参考方案1】：

好的，这就是我想出的。希望这可以帮助其他人： 我根据“类”（在我的例子中是种族）将托管对象解析为 NSMutableArrays。然后我将每个 NSM 数组放入一个 NSMutableDictionary 中，其中索引充当键。这个字典和其中的数组就成为确定节数和节中行数的基础。

- (void)setupFetchedResultsController // attaches an NSFetchRequest to this UITableViewController

MarksFromMeets* results = nil;

NSMutableArray * bestMarks = [[NSMutableArray alloc] init];
NSMutableDictionary * dictionaryOfMarksByRace = [[NSMutableDictionary alloc] init];

NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"MarksFromMeets"];

//    NSLog(@"self.athlete.athleteID = %@",self.athlete.athleteID);

request.predicate = [NSPredicate predicateWithFormat:@"(ANY whoRan.athleteID in %@) AND (eventPR > 0)",
                     self.athleteIDsForPredicate];
/* Call the records for selected athletes (PLURAL) */


request.sortDescriptors = [NSArray arrayWithObjects:
                           [NSSortDescriptor sortDescriptorWithKey:@"event" ascending:YES],
                           [NSSortDescriptor sortDescriptorWithKey:@"whoRan.athleteID" ascending:NO],
                           [NSSortDescriptor sortDescriptorWithKey:@"sortMark" ascending:YES],
                           nil];


NSArray * allMarks = [self.headToHeadManagedObjectContext executeFetchRequest:request error:nil];

MarksFromMeets* previousMark;

int i = 0;
int j = 0;
for (MarksFromMeets* athletePRs in allMarks) 
    NSArray* sortedMarks;
    if (previousMark == nil) 
        previousMark = athletePRs;
     else 
        if ([athletePRs.event isEqualToString:previousMark.event]) 
            if ([athletePRs.whoRan.athleteID isEqualToString:previousMark.whoRan.athleteID]) 
                NSLog(@"athletePRs.%@ = previousMark.%@",athletePRs.sortMark,previousMark.sortMark);


                if ([athletePRs.sortMark intValue] < [previousMark.sortMark intValue]) 
                    previousMark = athletePRs;
                    NSLog(@"Should NEVER happen");
                
             else 
                [bestMarks addObject:previousMark];
                NSLog(@"Adding %@, %@, %@ to bestMarks for i = %d",previousMark.event, previousMark.markInEvent, previousMark.whoRan.athleteFullName,i);
                previousMark = athletePRs;
                NSLog(@"tempMark record now %@",previousMark.whoRan.athleteFullName);
                if (j == [allMarks count]-1) 
                    [bestMarks addObject:previousMark]; //Now tempMark is last MarksForMeet object
                    sortedMarks = [self sortNSMutableArray:bestMarks];
                    [dictionaryOfMarksByRace setObject:sortedMarks forKey:[NSNumber numberWithInt:i]];
                    bestMarks = [[NSMutableArray alloc] init];

                
            
         else 
            [bestMarks addObject:previousMark];
            previousMark = athletePRs;
            sortedMarks = [self sortNSMutableArray:bestMarks];
            [dictionaryOfMarksByRace setObject:sortedMarks forKey:[NSNumber numberWithInt:i]];
            NSLog(@"Adding %@ to bestMarks for i = %d",previousMark.event,i);
            bestMarks = [[NSMutableArray alloc] init];
            // reallocate bestMarks to ensure values in NSMutableDictionary are unique
            i++;
            if (j == [allMarks count]-1) 
                [bestMarks addObject:previousMark];
                sortedMarks = [self sortNSMutableArray:bestMarks];
                [dictionaryOfMarksByRace setObject:sortedMarks forKey:[NSNumber numberWithInt:i]];
                bestMarks = [[NSMutableArray alloc] init];
                // reallocate bestMarks to ensure values in NSMutableDictionary are unique
            
        
    
    j++;



[self setResultsDictionary:dictionaryOfMarksByRace];

过滤数据后不得不采取措施。可能有一种更有效的方法，但这种蛮力方法有效，因为我在这种方法中从 Core Data 中提取的记录相对较少。