核心数据获取记录按关系分组？

我有两个名为Expenses和ExpensesCategory的实体， ExpensesCategory与CategoryExpenses有以下属性 – name-expenseCategory，Destination – ExpensesCategory，inverse-expense。 ExpensesCategory有一个名为expenseCategory的字符串类型属性， Expenses有一个名为amount的属性。 现在我想获取费用由amountCategory分组的费用。 这是我的代码

 NSSortDescriptor *sortTitle =[NSSortDescriptor sortDescriptorWithKey:@"addedTimestamp" ascending:YES]; NSArray *sortDescriptors = [NSArray arrayWithObjects:sortTitle, nil]; NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init]; NSEntityDescription *entity = [NSEntityDescription entityForName:@"Expense" inManagedObjectContext:context]; NSPredicate *predicate; predicate =[NSPredicate predicateWithFormat:@"(addedTimestamp = %@)",currentDate, endDate]; NSAttributeDescription* amtDescription = [entity.attributesByName objectForKey:@"amount"]; NSRelationshipDescription* nameDesc = [entity.relationshipsByName objectForKey:@"expenseCategory"]; NSExpression *keyPathExpression = [NSExpression expressionForKeyPath: @"amount"]; NSExpression *countExpression = [NSExpression expressionForFunction: @"sum:" arguments: [NSArray arrayWithObject:keyPathExpression]]; NSExpressionDescription *expressionDescription = [[NSExpressionDescription alloc] init]; [expressionDescription setName: @"Sum"]; [expressionDescription setExpression: countExpression]; [expressionDescription setExpressionResultType: NSDecimalAttributeType]; [fetchRequest setPropertiesToFetch:[NSArray arrayWithObjects:amtDescription, expressionDescription, nameDesc, nil]]; [fetchRequest setPropertiesToGroupBy:[NSArray arrayWithObject:nameDesc]]; [fetchRequest setResultType:NSDictionaryResultType]; [fetchRequest setEntity:entity]; [fetchRequest setPredicate:predicate]; [fetchRequest setSortDescriptors:sortDescriptors]; [context executeFetchRequest:fetchRequest error:&error]; 

但是当我运行Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Attribute/relationship description names passed to setPropertiesToFetch: must match name on fetch entity得到Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Attribute/relationship description names passed to setPropertiesToFetch: must match name on fetch entity

任何帮助，将不胜感激。

Answer 1

以下是您的问题的答案。

 NSEntityDescription *entity = [NSEntityDescription entityForName:@"ExpenseCategory" inManagedObjectContext:context]; NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init]; [fetchRequest setEntity:entity]; NSError *error; fetchRequest.predicate =[NSPredicate predicateWithFormat:@"(addedTimestamp <= %@) AND (addedTimestamp >= %@)",currentDate, endDate]; NSSortDescriptor *sortTitle =[NSSortDescriptor sortDescriptorWithKey:@"addedTimestamp" ascending:YES]; fetchRequest.sortDescriptors = [NSArray arrayWithObjects:sortTitle, nil]; NSExpressionDescription *ex = [[NSExpressionDescription alloc] init]; [ex setExpression:[NSExpression expressionWithFormat:@"@sum.expense.amount"]]; [ex setName:@"Sum"]; [ex setExpressionResultType:NSDecimalAttributeType]; [fetchRequest setPropertiesToFetch:[NSArray arrayWithObjects:@"expenseCategory", ex, nil]]; [fetchRequest setPropertiesToGroupBy:[NSArray arrayWithObject:@"expenseCategory"]]; [fetchRequest setResultType:NSDictionaryResultType ]; // Execute the fetch. NSArray *matchingObjects = [context executeFetchRequest:fetchRequest error:&error]; if ([matchingObjects lastObject]) { for (id expenseCategory in matchingObjects) { NSLog(@"%@", expenseCategory); } } 

如果您想要按Expense addedTime而不是ExpenseCategory addedTime进行过滤，则将谓词更改为：

 fetchRequest.predicate =[NSPredicate predicateWithFormat:@"(expense.addedTimestamp <= %@) AND (expense.addedTimestamp >= %@)",currentDate, endDate]; 

Answer 2

在回答另一个问题的同时，我发现了这个问题中突出显示的特定错误的原因（“传递给setPropertiesToFetch的属性/关系描述名称：必须与获取实体上的名称匹配”）。 我正在添加这个答案，以帮助那些寻求解决该错误的人。

出现此错误的原因是fetchRequest的propertiesToFetch正在fetchRequest的entity之前设置。 更改代码的顺序应解决问题：

  [fetchRequest setEntity:entity]; [fetchRequest setPropertiesToFetch:[NSArray arrayWithObjects:amtDescription, expressionDescription, nameDesc, nil]]; [fetchRequest setPropertiesToGroupBy:[NSArray arrayWithObject:nameDesc]]; [fetchRequest setResultType:NSDictionaryResultType]; 

Apple文档很清楚， propertiesToFetch包含的属性和关系必须与属性名称匹配：

“属性或关系描述的名称必须与获取请求的实体上的描述名称相匹配。”

但是我花了很长时间才意识到一旦你设置了propertiesToFetch （而不是执行fetch）就进行了这个测试，因此需要先设置entity 。

Answer 3

你正在传递|expressionDescription| ，这是一个NSExpressionDescription ，to setPropertiesToFetch: ; setPropertiesToFetch:接受只有NSPropertyDescriptions的数组，因此抛出exception。

在setPropertiesToGroupBy:允许使用NSExpressionDescriptions和NSPropertyDescriptions setPropertiesToGroupBy:但是。

Answer 4

你应该看一下预取技术。 在这个url中，您可以找到Employee和Department之间的示例。

希望有所帮助！