Add operator support to Timestamptz updates

[theelderbeever]

Similar to how it is shown in the docs for numeric types it would be awesome to be able to perform update operations using Timestamptz columns.

class Tasks(Table):
  started_at = Timestamptz()
  completed_at = Timestamptz(null=True)
  runtime = Numeric(null=True)

await Tasks.insert(Tasks(started_at=datetime.now()))

completed_at = datetime.now()
await Tasks.update(
  {
    "completed_at": completed_at, 
    "runtime": (completed_at - Tasks.started_at).total_seconds()}
)

So the basic idea being that updates treat values as datetime objects possibly? Maybe there is a separate way around this.

[dantownsend]

It's a good idea. The easiest to implement initially would be adding / substracting timedelta:

await Task.update({Task.completed_at: Task.completed_at + datetime.timedelta(days=1)})

[dantownsend]

I've made a prototype for this - #486

[dantownsend]

I've implemented a first version of this - will release it now.