核心数据:通过特定属性获取(连接关系)

  NSString *keyValue = @"title";
  NSString *valueValue = @"home";

  NSFetchRequest *request = [[NSFetchRequest alloc] init];
  [request setEntity:[NSEntityDescription entityForName:@"Page" inManagedObjectContext:_context]];
  [request setPredicate:[NSPredicate predicateWithFormat:@"(SUBQUERY(attributes, $a, $a.key == %@ && $a.value == %@).@count != 0)", keyValue, valueValue]];

更简单的谓词 ANY attributes.key = "title" AND ANY attributes.value = "home" 不起作用,因为它还返回有两个dict的页,例如key='addr'/value='home'和key='title'/value='pete'。

假设你从知道字典的“键”开始,输入“标题”。

创建NSPredicate时,为什么不使用已知的“键”在DictionaryEntry对象上创建NSFetchRequest呢。

    NSFetchRequest *request = [[NSFetchRequest alloc] init];
[request setEntity:[NSEntityDescription entityForName:@"DictionaryEntry" inManagedObjectContext:_context]];
[request setPredicate:[NSPredicate predicateWithFormat:@"key == %@", keyValue]];