EOExpressionArray
From EOFWiki
An EOExpressionArray is an internal class, that is used to store key paths in a more convenient fashion. The contents of an EOExpressionArray are invariabley EORelationships and EOAttributes.
So a key path @"address.country.name" would be an EOExpressionArray containing the EORelationships named "address" and named "country" and the EOAttribute with the name "name".