create or replace function fsm.is_valid_transition(shard bigint, machine_id bigint, event_ text)
  returns bool as
$$
  begin
      perform true
      from fsm.state_machine_state current_state

      join fsm.state child_state
          on  child_state.statechart_id = current_state.statechart_id
          and child_state.id = current_state.state_id

      join fsm.state state
          on  state.statechart_id = current_state.statechart_id
          -- is state.node_path an ancestor of current_state.node_path (or equal)?
          -- this is used to get all of the current_state parents, including the current_state
          and state.node_path @> child_state.node_path

      -- check if the transition named "event_" is a valid one for
      -- the current_state. That is, we need to see if "current_state"
      -- appears as a "source_state" in the transition table and has
      -- an associated "event" name equals to event_
      join fsm.transition transition
          on transition.statechart_id = state.statechart_id
          and transition.source_state = state.id
          and transition.event = event_

      where   current_state.shard_id = shard
          and current_state.state_machine_id = machine_id
          and current_state.exited_at is null;

      if not found then
          return false;
      end if;

    return true;
  end;
$$ language plpgsql stable strict;