Idiot's Guide to SSAS Attribute Relationships

Yes, that's me - and this is a reference post because frankly, this stuff is too complicated to keep in my head for how frequently I use it.

The Problem

When you get the "Errors in the OLAP storage engine: The attribute key cannot be found when processing." error returned when you're trying to process an SSAS cube, what does that really mean? I have a previous post where I went into some more technical reasons that can cause this error, but there's usually a pretty simple cause related to how you've structured your dimension in the designer... your attribute relationships.