问题
I have two entities which are related by a many-to-many relationship:
Database <<----->> Category
In other words, a database can have many categories and a category can be associated with many databases.
I need an NSPredicate that will return all Category objects associated with a given database object. Any help would be appreciated.
回答1:
You don't need a predicate. Given that you have a relationship called categories that is the to-many relationship to your Category entities, then
NSSet *categoriesForDatabase = database.categories;
If you really want to use a predicate then it would be:
[NSPredicate predicateWithFormat:@"ANY databases = %@", database];
where "databases" is the name of the to many relationship on the Category entity and database is an instance of a Database entity.
回答2:
You want to compare a collection (all Category objects) to a given object, you can try something like this :
[NSPredicate predicateWithFormat:@"ANY categories = %@", category];
or
[NSPredicate predicateWithFormat:@"ANY databases = %@", database];
来源：https://stackoverflow.com/questions/19155743/core-data-many-to-many-predicate