核心数据获取[_NSObjectID_48_1 userName]：无法识别的选择器发送到实例 [英] Core Data getting [_NSObjectID_48_1 userName]: unrecognized selector sent to instance

问题描述

A - >> B

我有两个实体A& B，每个实体都是一对多关系。我试图通过分组访问特定的B属性值和所有A实体属性。但是我收到错误，因为 [_ NSObjectID_48_1 userName] ：无法识别的选择器发送到实例当我尝试访问B的关系对象（即a）。

  NSManagedObjectContext * context = [[SamCoreDataHelper sharedInstance] managedObjectContext]; 
 
 NSFetchRequest * fr = [[NSFetchRequest alloc] initWithEntityName：@B]; 
 NSError * error; 
 NSExpressionDescription * total = [[NSExpressionDescription alloc] init]; 
 [total setExpression：[NSExpression expressionWithFormat：@@ sum.marks]]; 
 [total setName：@total]; 
 [total setExpressionResultType：NSDecimalAttributeType]; 
 
 
 [fr setPropertiesToFetch：[NSArray arrayWithObjects：@subject，@a，total，nil]]; 
 [fr setPropertiesToGroupBy：[NSArray arrayWithObjects：@subject，@a，nil]]; 
 [fr setRelationshipKeyPathsForPrefetching：[NSArray arrayWithObject：@a]]; 
 fr.returnsObjectsAsFaults = NO; 
 [fr setResultType：NSDictionaryResultType]; 
 
 NSArray * resultArray = [context executeFetchRequest：fr error：& error]; 
 
 for（NSDictionary * result in resultArray）{
 NSNumber * total = [result valueForKey：@total]; 
 NSString * subject = [result valueForKey：@subject]; 
 A * a = [result objectForKey：@a]; 
 NSLog（@％@，a.useName）; //这里我得到错误为** [_ NSObjectID_48_1 userName]：无法识别的选择器发送到实例** 
} 
  




解决方案

您将收到object id当 NSFetchRequest 的结果设置为 NSDictionaryResultType 时，相对对象的NSManagedObject p>



所以你需要通过对象id来获取它：

  A * a =（A *）[managedObjectContext objectWithID：[result valueForKey：@a]]; 
 NSLog（％@，a.userName）; 
  

UPD：

  fr.returnsObjectsAsFaults = NO; 
 

在这里是无用的，因为返回类型是 NSDictionaryResultType 。

A->>B

I have two entities A&B and each are related as one to many relationship. I am trying to access specific B attribute value and all A entity attributes by grouping. However I am getting error as [_NSObjectID_48_1 userName]: unrecognized selector sent to instance when I try to access relationship object of B(that is a).

NSManagedObjectContext *context = [[SamCoreDataHelper sharedInstance] managedObjectContext];

    NSFetchRequest *fr = [[NSFetchRequest alloc] initWithEntityName:@"B"];
    NSError *error;
    NSExpressionDescription *total = [[NSExpressionDescription alloc] init];
    [total setExpression:[NSExpression expressionWithFormat:@"@sum.marks"]];
    [total setName:@"total"];
    [total setExpressionResultType:NSDecimalAttributeType];


    [fr setPropertiesToFetch:[NSArray arrayWithObjects:@"subject",@"a", total, nil]];
    [fr setPropertiesToGroupBy:[NSArray arrayWithObjects:@"subject", @"a", nil]];
    [fr setRelationshipKeyPathsForPrefetching:[NSArray arrayWithObject:@"a"]];
    fr.returnsObjectsAsFaults = NO;
    [fr setResultType:NSDictionaryResultType ];

    NSArray *resultArray = [context executeFetchRequest:fr error:&error];

for (NSDictionary *result in resultArray) {
        NSNumber *total = [result valueForKey:@"total"];
        NSString *subject = [result valueForKey:@"subject"];
        A *a = [result objectForKey:@"a"];
        NSLog(@"%@", a.useName);// Here I am getting error as **[_NSObjectID_48_1 userName]: unrecognized selector sent to instance**
}

解决方案

You will receive "object id" instead of instance of NSManagedObject for the relative object when the result of NSFetchRequest is set to NSDictionaryResultType.

So you need to fetch it "by object id":

A * a = (A*)[ managedObjectContext objectWithID:[result valueForKey:@"a"]];
NSLog("%@",a.userName );

UPD:

fr.returnsObjectsAsFaults = NO;

is useless here because the return type is NSDictionaryResultType.

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