Quick update for you: assuming

document:document1#admin@user1:user

and

document:document1#member@user1:user

should be mutually exclusive, writing each relationship in succession causes the second write to fail the precondition as expected (without having to specify an OptionalRelationship in the precondition filter). However, if I send both relationships in one write with the precondition it succeeds. Not sure if that's by design due to when preconditions are checked. Additionally, if one relationship in a group of writes fails, should all writes fail, or does it depend on whether or not the dataStore has transaction mechanics?