The API will translate the request into a command message and dispatch the message on the command bus. The command bus is responsible to locate and invoke the responsible command handler. To do so the command bus also need some route information.
In the command handler you should load the effected Todo from the TodoList
The Todo aggregate root is responsible for handling the action so it needs a new method.
In the method the Todo should check it's current status. Only if it is done it can be marked as open again otherwise it should throw an exception
If the precondition is met the Todo should not change it's state directly but instead record a new domain event called TodoWasReopened
Add a state changing method in the Todo aggregate root based on the new event. Call the method whenTodoWasReopened and require the recorded event as a input parameter then change the TodoStatus to open
The same event needs also be handled by a projector so that the read model gets updated. You already know how to route a command message to a handler. Now do the same for the event and route it to a projector which updates the read model table.
Bonus Points: Write at least one unit test which covers that a done Todo can change its state to open again and records a TodoWasReopened event to track the change.
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