是否可以在核心数据查询中仅获取选定的属性 [英] Is it possible to fetch only selected property in a Core Data query

问题描述

我有(我猜)典型的问题.我的 Core Data 数据库包含表，其中包含 BLOB 字段和相当长的文本字段.

I have (I guess) typical problem. My Core Data database contains table, which have BLOB fields and quite long text fields.

通常我不需要加载那些大尺寸的字段.由于数据库中有数千条记录，我更愿意只获取我真正需要的数据.换句话说，我想进行SELECT name, id FROM TAB_NAME"查询，而不是SELECT * FROM TAB_NAME"查询.

Usually I do not need to load agrresivly those large size fields. Since there are several thousand records in a database I would prefer to fetch only data I truly need. In other words I would like to make "SELECT name, id FROM TAB_NAME" query, not "SELECT * FROM TAB_NAME" query.

有什么方法可以使用 Core Data 只获取选定的字段?或者我应该寻找其他选项，例如，将大字段分开到另一个表?也许有更好(更简单)的方法来做到这一点?

Is there any way to fetch only selected fields using Core Data? Or shall I look for some other option, like, for instance, seperate large fields to another table? Maybe there is a better (easier) way to do this?

我习惯于使用 Hibernate 或 JPA，在这些地方很容易完成上述操作，但在阅读 Core Data 文档后，我没有看到这样的选项.

I am used to working with Hibernate or JPA, where it easy to do described above operation, after going through Core Data docs I do not see such option.

推荐答案

换句话说，我想做SELECT name, id FROM TAB_NAME"查询，而不是SELECT * FROM TAB_NAME"查询.

In other words I would like to make "SELECT name, id FROM TAB_NAME" query, not "SELECT * FROM TAB_NAME" query.

这个问题的答案已被接受，但仅供参考，以下是如何仅获取具有核心数据的实体的特定属性，而无需重构您的模型:

The answer to this question was accepted, but FYI, here's how to fetch only specific properties of an entity with Core Data, without re-factoring your model:

// build your NSFetchRequest
// ...

[request setResultType:NSDictionaryResultType];
[request setPropertiesToFetch:
  [NSArray arrayWithObjects:@"property1", @"property2", /* etc. */ nil]];    

另请参阅 NSFetchRequest.

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