Possible authorization logic bug in assignments#show

[s-edwards]

The assignments#show action requires the current user to have edit access to all offerings of the currrent assignment, or a CanCan exception will be thrown. But what if an instructor for this term's offering of course X is not an instructor for other (older) offerings of course X, and so does not have edit access to assignment offerings under those prior course offerings? This appears to make assignments created by earlier instructors unusable/uneditable by a new instructor?