核心数据一对多关系：获取特定的相关实体 [英] Core data one to many relationships: fetching specific related entities

问题描述

我有2个实体：火车和车站

每列火车都在多个车站停靠，即火车< --->站。
火车停靠的站点列表与其他火车停靠的站点列表不同。

Each train stops at multiple stations, i.e. Train <--->> Station. The list of stations a train stops at is different from the list of stations some other train stops at.

如何正确保存并获取列表

How do I correctly save and fetch the list of stations for a particular train?

谢谢

推荐答案

这是吗？

Train <--->> Station

意味着您有一对多的关系？如果是这样，除非每个站只能停一列火车，否则它是行不通的。我要做的是创建第三个实体，称其为停止，具有两个关系，一对多到火车，多对一个到 Station 。看起来就像

mean you have a one to many relationship? If so, it doesn't work, unless only one train can stop at each station. What I would do is create a third entity, call it say "stop" with two relations, a many to one to Train and a many to one to Station. It wil look something like

+-----+       +-------+
|Train|       |Stop   |        +-------+
+-----+       +-------+        |Station|
|stops|<---->>|train  |        +-------+
+-----+       |station|<<----->|stops  |
              +-------+        +-------+

要查找特定火车停靠的所有车站，只需查看火车的车站属性即可， 停止对象的 NSSet 个对象，每个对象都有一个属性，该属性是火车停靠的车站。

To find all the stations a particular train stops at, just look at the stops property of the Train which will be an NSSet of Stop objects each of which has a property which is a station the train stops at.

要添加新的停靠点，只需创建一个新的停止对象并设置其火车和站属性。

To add new stops, just create a new Stop object and set its train and station properties appropriately.

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