使用谓词搜索核心数据实体
我正在编写一个使用 Core Data 作为其持久层的 Mac 应用程序，并且很好奇搜索的“所有权”的普遍共识是什么。
假设我有一个名为 Recipe 的类（NSManagedObject 的生成子类），并且我希望能够搜索“包含某种成分的食谱”、“可以喂饱 4 人以上的食谱”等等。我可以设想这样的方法：
-(NSArray *)fetchRecipesContaining:(Ingredient *)ingredient;
-(NSArray *)fetchRecipesFeedingMorePeopleThan:(int)count;
并且实现知道如何构造适当的 NSFetchRequest/NSPredicate 来获取它们。问题是什么对象通常实现这些方法？在 Java 世界中，这些通常存在于 RecipeFactory 上的实例方法中。我还可以看到 Recipe 类中的类方法的情况。
在 Hello World 应用程序（大多数教程似乎都提供）中，逻辑直接嵌入到调用代码中，但是，我并不热衷于将处理 NSFetchRequests 和 NSEntityDescriptions 的样板代码分散在整个调用代码中。我真的更愿意抽象出该逻辑，并允许使用更有意义的 API（如上所述）来获取对象。
我倾向于在 Recipe 上使用类方法（并使用类别实现，这样我就不必修改生成的类），但想将其扔在那里，看看其他核心数据用户可能会使用什么。
谢谢。
I am writing a Mac app that uses Core Data as its persistence layer, and am curious as to what the general consensus is with the "ownership" of searches.
Let's say that I have a class called Recipe (generated subclass of NSManagedObject) and I would like to be able to search for, say, "Recipes containing a certain ingredient", "Recipes that can feed more than 4 people", and so on. I can conceive methods like:
-(NSArray *)fetchRecipesContaining:(Ingredient *)ingredient;
-(NSArray *)fetchRecipesFeedingMorePeopleThan:(int)count;
and the implementation knows how to construct an appropriate NSFetchRequest/NSPredicate to go get them. The question is what object normally implements these methods? In a Java world, these would often live in an instance method on a RecipeFactory. I can also see the case for a class method in the Recipe class.
In a Hello World application (which most tutorials seem to offer) the logic is embedded directly in the invoking code, however, I am not that keen on having boilerplate code dealing with NSFetchRequests and NSEntityDescriptions scatted all through the invoking code. I really would prefer to abstract that logic away and allow a more meaningful API (per above) for fetching objects.
I am inclining towards a class method on Recipe (and implemented using categories so that I don't have to modify the generated classes), but wanted to throw it out there and see what other Core Data users might use.
Thanks.
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使用类别。正如您所注意到的，这使 Xcode 生成的类保持不变，同时允许您扩展托管对象类的功能。
Use categories. As you note, this leaves the Xcode-generated classes untouched while allowing you to extend the functionality of the managed object class.
我正在使用类方法的方法。我不确定它是否好，但还没有遇到任何麻烦。 Ruby on Rails 也使用它的 ActiveRecord 来到 Cocoa/CoreData - 这样做似乎很合乎逻辑，因为它在 ActiveRecord 库中也以相同的方式实现。
I'm using the approach with class methods. I wasn't sure if it's good, but haven't had any trouble with it yet. Also coming to Cocoa/CoreData from Ruby on Rails with it's ActiveRecord - it seemed only logical to do so, because it's implemented the same way in ActiveRecord library as well.