How to set config.assets_dir? (Rails 3.0.9)

Hi,

I have a problem with Rails 3.0.9. I am generating some concatenated
javascript files with a runner, it is working flawlessly in Rails 2.3.
With Rails 3.0.9 I get an Exception with this backtrace:

actionpack-3.0.9/lib/action_view/helpers/asset_tag_helper.rb:794:in
`rails_asset_id'
...
railties-3.0.9/lib/rails/commands/runner.rb:50:in `eval': can't
convert nil into String (TypeError)

The rails_asset_id method is using config.assets_dir to join it with
the javascript file name, but config.assets_dir is nil and it fails.
I've tried to set the assets_dir in various places, but without
success. I also can't find any documentation for this parameter.

Does anybody has a clue, how one should set the assets_dir?

Thanks,

r.

@Rudolf_Gavlas Any solution you found facing same issue