It should be possible to move AssociationKind and collectionRole should from AssocationKey to AssocationKeyMetadata as they are constant for a given key family. If so, we may save the repeated re-calcuation and the hard-to-grasp and -maintain code in AssociationPersister may be simplified.

Also it may be possible to avoid the repeated instantiation of AssocationKeyMetadata in EntityDehydrator by obtaining them from the corresponding collection/entity persisters.