Set active class on parrents in a structure?

Question

I have a section (structure) called "blog" but "blog" is not in the URL.

I have entries like:

Entry 1a
--Entry 1b
----Entry 1c
Entry 2a
---Entry 2b

A navigation like:

Overview <- a template in the root directory
Entry 1a
Entry 2a

I need to set "Overview" to active on all entries. Entry 1a should be active if itself or child's are shown. Entry 2a should be active if itself or child's are shown.

Doing this by hand for all entries would be a pain:

{% if craft.request.firstSegment == 'Entry 1a or Entry 1b' %}active{% endif %}