Question

我已经在这里和所有地方阅读了我在这个主题上发现的所有内容，但仍然无法弄清楚如何做我需要的。

我在核心数据db中有一个简单的事件对象，其属性如title, status, startDate, endDate, dayDate，其中status是整数，有3个可能的值，dayDate是没有时间的日期（或午夜日期）

我需要收集有关数据库中事件的一些统计信息，按天分组并具有不同状态的事件计数。在示例中，我需要像伪字典这样的结果：

`

[
@{
    @"dayDate" : ...someDate..
    @"eventsCountTotal" : 8,
    @"eventsWithStatus1" : 4,
    @"eventsWithStatus2" : 3,
    @"eventsWithStatus3" : 1
}
@{
    ...stats for another day
}
]

`

使用CoreData完全可以使用一个查询吗？我目前正在做这个以获得一些统计数据，但这不是我想要的：

    NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"SWEvent"];
    request.resultType = NSDictionaryResultType;

    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"dayDate < %@", [NSDate todayDate]];

    NSExpression *keyPathExpression = [NSExpression expressionForEvaluatedObject];
    NSExpression *countExpression = [NSExpression expressionForFunction:@"count:" arguments:@[keyPathExpression]];

    NSExpressionDescription * expressionDescription = [NSExpressionDescription new];
    expressionDescription.name = @"eventsCount";
    expressionDescription.expression = countExpression;
    expressionDescription.expressionResultType = NSInteger32AttributeType;

    NSEntityDescription *entity = [NSEntityDescription entityForName:@"SWEvent" inManagedObjectContext:self.managedObjectContext];

    NSDictionary *attributes = [entity attributesByName];
    NSAttributeDescription *statusDescription = attributes[@"status"];
    NSAttributeDescription *dayDateDescription = attributes[@"dayDate"];

    NSSortDescriptor *sortDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"dayDate" ascending:NO];

    request.predicate = predicate;
    request.propertiesToFetch = @[calledDescription, dayDateDescription, statusDescription];
    request.propertiesToGroupBy = @[dayDateDescription, statusDescription];
    request.sortDescriptors = @[sortDescriptor];

    NSError *error = nil;

    @try {
        NSArray *result = [self.managedObjectContext executeFetchRequest:request error:&error];
    }
    @catch(NSException *exception) {

        NSLog(@"Exception occurred: %@, %@", exception, [exception userInfo]);
    }

但是这段代码给了我这个，这显然是我不想要的，需要进一步过滤： `

{
    status = 2;
    eventsCount = 1;
    dayDate = "2015-07-27 00:00:00 +0000";
},
{
    status = 1;
    eventsCount = 2;
    dayDate = "2015-07-26 00:00:00 +0000";
},
{
    status = 2;
    eventsCount = 3;
    dayDate = "2015-07-26 00:00:00 +0000";
}

`

有没有办法如何在一个查询中实现我需要的东西，或者我真的不能做得比这更好？

非常感谢

Answer 1

这种情况的方法是在内核中执行，如“核心数据编程指南”中所建议的那样。我认为这会导致代码明显减少。如果你遇到任何性能或内存瓶颈会很有趣，但我会怀疑它。（这应该在具有x0000记录的旧硬件上运行良好。）

NSArray *allDates = [allEvents valueForKeypath:@"dayDate"];
NSMutableArray *result = [NSMutableArray array];
for (NSDate *date in allDates) {
   NSMutableDictionary *summary = [NSMutableDictionary dictionary];
   NSArray *dayRecords = [allEvents filteredArrayUsingPredicate:
       [NSPredicate predicateWithFormat:@"dayDate = %@", date]];
   [summary addObject:@{"dayDate" : date}];
   [summary addObject:@{"eventsCountTotal" : dayRecords.count}];
   for (int x = 1; x <= 3; x++) {
      NSArray *subgroup = [dayRecords filteredArrayUsingPredicate:
          [NSPredicate predicateWithFormat:@"status = %i", x]];
      [summary addObject:
          @{[NSString stringWithFormat:@"status%iCount", %@] : @(subgroup.count)}]
   }
   [result addObject:summary]
}

CoreData与子表达式聚合在一起

1 个答案: