核心数据中的多个逆关系
我有一个实体图书馆，包含两个图书列表。图书馆维护这两个图书清单非常重要。在我的图书馆实体上，我有一个从每个列表到我的图书实体的一对多关系。同样，Book也有一个关系“图书馆”。我的数据从数据库中删除时遇到一些问题，我读到我应该设置反向关系以帮助保证数据完整性。然而，在这种情况下，一本书希望能够与我的图书馆实体上的每个列表建立反向关系。我怎样才能做到这一点？
我天真的第一个想法是实现两个列表的关系。因此，一本书有一个关系“libraryForList1”和“libraryForList2”，这样每个关系都可以有一个逆关系。我永远不必实际引用这些属性，因为根据核心数据规范，如果我将一本书添加到其中一个图书馆列表中，它会自动将图书馆设置为该图书的所有者。
I have an entity Library that mains two lists of Books. It is important that the library maintain these two lists of books. On my Library entity I have a relationship thats one to many from each list to my Book entity. Likewise, Book has a relationship "library". I'm having some problems with my data being erased from the database and I read that I should be setting up inverse relationships to help with data integrity. In this case however, a Book would want to be able to set up an inverse relationship with each list on my Library entity. How can I accomplish this?
My naive first thought is to implement relationships for both lists. So a book has a relationship "libraryForList1" and "libraryForList2" so that it can have an inverse for each relationship. I'll never have to actually reference these properties because according to the Core Data spec, if I add a book to one of the library lists, it automatically takes care of setting the library as that books owner.
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您的“天真的第一个想法”本质上是正确的：如果您的图书馆实体与图书有两个一对多的关系（为了清楚起见，我们将关系称为“ownedBooks”和“borrowedBooks”），那么您的 Book 实体应该具有一对一的逆关系（
owningLibrary和borrowingLibrary），然后 Core Data 将使您的生活更轻松。您可能还需要考虑这些关系的删除规则：如果由于某种原因删除了一本书，则
owningLibrary和borrowingLibrary的删除规则可能是 无效 – 也就是说，两个图书馆都会从其列表中删除该书。删除仍然拥有图书的图书馆似乎是一个坏主意，因此也许ownedBooks和borrowedBooks的删除规则应该是拒绝：图书馆可以在所有书籍都被清点（并从图书馆中删除）之前不会被删除。Your "naive first thought" is essentially correct: If your Library entity has two one-to-many relationships with Books (for clarity let's call the relationships
ownedBooksandborrowedBooks), then your Book entity should have to-one inverse relationships (owningLibraryandborrowingLibrary) and then Core Data will make your life easier.You may also need to think about the delete rules on these relationships: If a Book is deleted for some reason, the delete rules for the
owningLibraryandborrowingLibrarywould likely be Nullify – that is, both libraries would remove the Book from their lists. Deleting a Library that still has Books seems like a bad idea, so maybe the delete rule forownedBooksandborrowedBooksshould be Deny: A Library can't be deleted until the books are all accounted for (and removed from the Library).