Excelent post regarding Creating 1-n and n-n relations: http://orchardproject.net/docs/Creating-1-n-and-n-n-relations.ashx
This explains Part -> Record and Record -> Record relations.
The one question I did have is how should this work for releated parts, Part -> Part relations?
i.e. Would it not be normal for a BlogPostPart to contain a reference to the BlogPart? so that the BlogPostPartRecord table contains a FK to the BlogPart or BlogPartRecord.
I'm creating a forum, hence a PostPart needs a reference to a TopicPart and ForumPart.