有没有人有一个核心数据中的获取属性的工作示例？ [英] does anyone have a working example of a fetched-property in core-data?

问题描述

我已经尝试使用抓取的属性几次，虽然它似乎是正确的方法，它永远不会工作

I have tried to use fetched properties a couple of times, and although it seems to be the right approach, it never works.

在我最近的尝试中，我添加了fetched属性到我的实体，选择模型中的其他实体作为目的地，并将谓词设置为我知道有效的条件。

In my latest attempt I added the fetched-property to my entity, selected the other entity in the model as the 'destination', and set the predicate to a condition that I know is valid.

问题1：当生成实体的NSManagedObject类时，它不包括fetched-property的任何内容。一些搜索后，我在.h文件中添加了声明，在.m文件中添加了@dynamic语句（是的，我知道它是一个NSArray *类型）。

Problem 1: When the NSManagedObject-class for the entity is generated it does not include anything for the fetched-property. After some searching I added the declaration for it in the .h file and the @dynamic statement for it in the .m file (yes, I know it's an NSArray * type).

问题2：即使之后，当我访问这个属性在代码中，我得到一个异常抛出的状态，大意是fetch-request没有一个实体。我假设实体是指定为目的地的实体，并且实际上是在那里。

Problem 2: Even after that, when I access this property in code I get an exception being thrown that states something to the effect that the fetch-request does not have an entity. I am assuming that the 'entity' would be the one specified as the 'destination' and it is, in fact, there.

所以，我想要一个人提供一个具体的工作示例（iOS平台），其中在模型中定义了一个获取的属性，在NSManagedObject派生类中声明，并实际从代码中使用。

So, I'd like someone to provide a concrete working example (iOS platform) where a fetched-property is defined in the model, declared in a NSManagedObject-derived class, and actually used from code.

推荐答案

这里是我的相关位（包括你已经提到的位）：

Here's my relevant bits of code (including bits you've already mentioned):

我的例子有一个'Card'对象，与'Stats'对象有1->多关系。每个Stats对象都有一个结果，可以是1-4。我获取的属性是一个简单的，给我的'卡'对象一个'统计'对象的数组，只有'结果'= 1。

My example has a 'Card' object that has a 1->many relationship with a 'Stats' object. Each 'Stats' object has an 'outcome' that can be 1-4. My fetched property is a simple one to give my 'Card' object an array of 'Stats' objects that are of 'outcome'=1 only.

获取的属性，以便我可以很容易地抓住具有超过一定数量和种类的'统计'对象的'卡'对象。

I wanted to use the fetched property so that I could easily get hold of 'Card' objects that had more than a certain number and kind of 'Stats' objects.

在这个获取的属性的谓词中，我放置了

In the predicate for this fetched property I put

(SELF.outcome=1) AND (SELF.card=$FETCH_SOURCE)

'SELF'是'stats'记录，$ FETCH_SOURCE在执行时成为'Card'对象。

'SELF' is the 'stats' record, and $FETCH_SOURCE magically becomes the 'Card' object when executed.

正如你所做的，我把'Card'对象的.h和.m文件中的以下内容：

As you did, I put the following in the .h and .m files for the 'Card' object:

@property (nonatomic, retain) NSArray *statsOfTypeOne;
@dynamic statsOfTypeOne;

然后在我的代码中我使用：

Then in my code I used:

[self.managedObjectContext refreshObject:cardInstance mergeChanges:YES];
[cardInstance valueForKey:@"statsOfTypeOne"]

.statsOfTypeOne应该很好）。没有刷新对象，它没有更新Fetched属性（根据手册）。

to get at the array (although cardInstance.statsOfTypeOne should be fine). Without the refresh object it wasn't updating the Fetched property (as per the manual).

我这是我做的一切这行得通。请告诉我是否适合您。

I think that's everything that I did to make it work. Let me know if it works for you.

彼得

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