核心数据获取与另一个对象关系的对象 [英] Core Data Fetching Objects That Are in Relationship to another object

问题描述

我想获取与对象A具有多对多关系的对象B（即A - >> B）。我已经记忆中有A.我知道我可以从A获得B对象的NSSet，但最好是对B对象进行获取请求（特别是如果我想在UITableView中按日期排序部分）？

I want to fetch objects B which has a to-many relationship with object A (ie. A -->> B). I already have A in memory. I know I could get a NSSet of the B objects from A, but would it be better to make a fetch request of the B objects (especially if I want to sort sections by date in a UITableView)?

我如何才能提出只获得属于单个对象A的B对象的请求？

How would I make such a request that only gets the B objects belonging to a single object A?

推荐答案

如果您已经持有A的实例，只需通过A的访问者访问相关的B实例。

If you are already holding an instance of A, just access the related B instances through A's accessor.

如果您需要直接获取与a相关的所有B特别是A（你不是在这种情况下），你要为B实体构建一个获取请求，其中一个谓词基于Bs与A的（反）关系。（具体语法将取决于反向关系名称，以及该反转是一对一还是多对。）

If you need to directly fetch all the B's related to a particular A (you don't in this case), you'd build a fetch request for the B entity, with a predicate based on the (inverse) relationship of Bs to A. (The specific syntax will depend on the inverse relationship name, and whether that inverse is a to-one or to-many.)

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