Action.left_outer_joins(:actions_users).where(user_id: 1)
select *
from actions left outer join actions_users
on actions_users.action_id = actions.id and actions_users.user_id = 1
where actions.user_id = 1
Хотя вы еще не просили об этом, ...
Action.left_outer_joins(:actions_users).where(actions_users: {status: 'active'})
select *
from actions left outer join actions_users
on actions_users.action_id = actions.id and actions_users.user_id = 1
where actions_users.status = 'active'