How to obtain relationship data?

I have some a couple of entities. One is used for the actual product information. The second one is for the information regarding certain documentation (in this case some PDF files). I am trying to pull the data of both the product and it's corresponding data in the document entity. Not all products have documents connected to them, so in a way I am trying to d开发者_Python百科o what would be a join in a relational database.

I also have declared relationships for both entities. I am relatively new using Core Data. I can fetch the data for the individual entities, but can't seem to have the same success for the relationships.

Does anyone know of a good tutorial, or can point me in the right direction to fetch the information only relevant to the relationship?

When setting up an NSFetchRequest, you can pass an NSArray of keys for the relationship you want to fetch using -setRelationshipKeyPathsForPrefetching: