如何获取在CoreData中取平均值的Entity属性

Posted 2023-03-13

【中文标题】如何获取在CoreData中取平均值的Entity属性

【英文标题】：How to get the Entity property from which an average is taken in CoreData

【发布时间】：2013-07-25 09:27:04

【问题描述】：

我从我的CoreData Entity、Averaged 和GroupedBy date 结果中提取了一些值。查询效果很好。现在...我想知道这些结果的来源dates。显示的输出来自原始结果。我怎么能让我的查询也吐出 date/GroupBy value (given that my GroupBy criteria is date) 从中获取这些平均值

输出

(
        
        averageWeight = 36;
    ,
        
        averageWeight = 22;
    
)

守则：

NSFetchRequest *request = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"ExerciseData" inManagedObjectContext:context];
    [request setEntity:entity];

    // Specify that the request should return dictionaries.
    [request setResultType:NSDictionaryResultType];

    // Create an expression for the key path.
    NSExpression *keyPathExpression = [NSExpression expressionForKeyPath:@"weight"];

    // Create an expression to represent the function you want to apply
    NSExpression *expression = [NSExpression expressionForFunction:@"average:"
                                                         arguments:@[keyPathExpression]];

    // Create an expression description using the minExpression and returning a date.
    NSExpressionDescription *expressionDescription = [[NSExpressionDescription alloc] init];

    // The name is the key that will be used in the dictionary for the return value.
    [expressionDescription setName:@"averageWeight"];
    [expressionDescription setExpression:expression];
    [expressionDescription setExpressionResultType:NSInteger32AttributeType]; // For example, NSDateAttributeType

    // Set the request's properties to fetch just the property represented by the expressions.
    [request setPropertiesToFetch:@[expressionDescription]];
    request.predicate = [NSPredicate predicateWithFormat:@"exercise == %@",exercise];
    [request setPropertiesToGroupBy:[NSArray arrayWithObject:@"date"]];
    // Execute the fetch.
    NSError *error;
    NSArray *objects = [context executeFetchRequest:request error:&error];
    NSLog(@"%@",objects);

【参考方案1】：

只需添加“日期”作为要获取的另一个属性：

[request setPropertiesToFetch:@[expressionDescription, @"date"]];