Another place that will need to be updated is this code: https://github.com/ash-project/ash_state_machine/blob/main/lib/ash_state_machine.ex#L140

Currently, we just check that the target state is in valid initial states. However, if the action is an upsert action, we now have to look up the valid next states for all transitions for this action and make sure that the given state is in that list.

Additionally, we have to ensure at compile time that a state transition listed for an upsert action has all states in its from list, because an upsert could update anything in any state. (this could be evolved later on)

Just pushed a commit for a compile time check that create ... upsert? true actions allow transitions from all states.

I'm having a hard time wrapping my head around this:

...if the action is an upsert action, we now have to look up the valid next states for all transitions for this action and make sure that the given state is in that list.

Did you mean "we have to look up the valid next states for all transitions for this resource"? In my current understanding each action can have only one transition.

If so does that mean we'd need to check for a configuration like this?

    transitions do
      transition(:begin, from: :pending, to: [:executing, :pending])
      transition(:complete, from: :executing, to: [:complete, :pending])
      transition(:initialize, from: :*, to: :pending)
      ...
    end

There can be multiple transitions per action. For example, you could have:

transition(:begin, from: :pending, to: :executing)
transition(:begin, from: :paused, to: :resumed)

This is different than

transition(:begin, from: [:pending, :paused], to: [:executing, :resumed])

because the latter allows going from paused to executing, whereas the former does not. So you'd look up the possible destinations for all transitions for this action, and make sure that the target is in one of the lists.

Alrighty, just pushed that!

@enoonan looks like a small credo issue. You can add @moduledoc false and then I'll merge this.

Added!