Add is_valid property to assignments?

[timwis]

Pulled out of #35

Use case:

Cathy and Anna are both Deputy Finance Directors. Contracts can be approved by either of them during the "Awaiting Finance Approval" state.

@mjumbewu said:

I'm considering adding an is_valid field to Assignment instances. So, in the case where Cathy and Anna are both Deputy Finance Directors, when one acts on their assignment, the other's assignment would get is_valid set to False -- perhaps explicitly in the handler, perhaps automatically as a consequence of the case transitioning out of the assigned state with no action taken on the assignment. Probably the latter.

Does that seem reasonable? If the case transitions to another state, then all incomplete assignments are simply marked as invalid?

@timwis replied:

maybe I'm misunderstanding, but I'm not sure that's necessary. If the /cases/awaiting/ endpoint queries cases that (a) are assigned to you, (b) you haven't acted on yet, and (c) the case is currently in the state you're assigned to -- it should handle both scenarios described above, no?