Core Data: Finding Related Objects

I am having some issue with understanding of Core Data. In my program there are two entities, say A and B. They have both references on each other and A has a to-many relationship to B.

I can have them displayed in TableViewControllers and I can display all As and all Bs. However, I want to only display all Bs that belong开发者_如何学C to a specific A, so I'd like to know the Core Data version of a where clause. As there are no foreign keys in Core Data, I guess it needs to go via the relationship, right?

I tried it with this:

NSPredicate *predicate = nil;
predicate = [NSPredicate predicateWithFormat:@"readingEntity.meter=%@",meter];

(in the function that I get all Bs, meter is my A).

Anybody out there who can help me on this one? Regards.

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You don't need a predicate at all if you already have an A object. Instead, you just ask the A object for its related B objects.

Don't think of Core Data as SQL. It's not. It works differently.