In the Project / Task option you can filter tasks by project.

However, I would like to be able to choose a project and seethe Tasks for all it and all of it's child projects.

I tried to edit the Search View, changing the project_id field filter to:

<field name="project_id" filter_domain="[('project_id','child_of',self)]"/>

But it doesn't work: no Tasks are shown with the filter.

I know that CRM opportunities uses a child_of filter on Partners just like this one, and it works. So. what might I be doing wrong?