我有两个模型 Practice 和 PracticeRecord ,它们之间存在一对多关系。现在我想检索一组 Practice 对象,其关联的 practiceRecord 对象的数量最小但大于零。我写了下面的代码,但它没有工作,并且在获取请求时发生了异常。任何人都可以给出一个优雅的解决方案。提前致谢。
NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"Practice"];
request.predicate = [NSPredicate predicateWithFormat:@"practiceRecords.@count > 0"];
[request setResultType:NSDictionaryResultType];
NSExpression *keyPathExpression = [NSExpression expressionForKeyPath:@"practiceRecords.@count"];
NSExpression *minExpression = [NSExpression expressionForFunction:@"min:" arguments:[NSArray arrayWithObject:keyPathExpression]];
NSExpressionDescription *expressionDescription = [[NSExpressionDescription alloc] init];
[expressionDescription setName:@"minPracticeRecordTimes"];
[expressionDescription setExpression:minExpression];
[expressionDescription setExpressionResultType:NSDecimalAttributeType];
[request setPropertiesToFetch:[NSArray arrayWithObject:expressionDescription]];
NSError *error = nil;
NSArray *objects = [context executeFetchRequest:request error:&error];
if (objects == nil) {
// Handle the error.
}
else {
if ([objects count] > 0) {
NSLog(@"Minimum practice record times: %@", [[objects objectAtIndex:0]
valueForKey:@"minPracticeRecordTimes"]);
}
}