问题描述
我正在使用一个使用Core Data的iPhone应用程式。我只有一个实体叫做Books，该实体具有Title，Author，ISBN，Description和Shelf的属性。
I'm working on an iPhone app that uses Core Data. I have only one entity called Books, and that entity has the attributes Title, Author, ISBN, Description and Shelf.
我正在编辑图书视图，想要有一个UIPickerView列出所有独特的Shelf行，以便用户可以选择一个货架。
I'm working on the book editing view, and want to have a UIPickerView listing all of the unique Shelf rows so that the user can just pick a shelf.
我的问题是 - 如何获得一个NSArray的所有的数据库中所有书的独特Shelf属性？我可以访问正在编辑的Book的managedObjectContext，所以我会使用某种获取请求吗？
My question is -- how do I get an NSArray of all of the unique Shelf attributes across all of the books in the database? I have access to the managedObjectContext of the Book being edited, so would I use some sort of a fetch request?
谢谢！
推荐答案
您可以使用 @distinctUnionOfArrays 。
但IMO将是一个更清洁的解决方案来重新设计您的数据模型，以便有两个实体（书和货架）。
然后您可以创建书和货架之间的关系。
You can get a duplicate-less array of available shelves with the array operator @distinctUnionOfArrays.
But IMO it would be a cleaner solution to redesign your data model, so that there are two entities (books and shelves).
You could then create a relationship between book and shelf.
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