combobox and m:n relation


i have a project and a task. both have a m:n relation to each other. I
made a scaffolding and want to insert a combobox in the task new view

Code : ruby - fold - unfold

    <label for="project_name_ger">Poject</label><br/>
        <%= select "task[projects_id][]", "project_id",
             Project.find(:all).collect {|c| [c.name_ger,]},
             :prompt => "Select a Category" %>

the code does not work. does anyone has an idea how to save the
combobox's content. it works for a 1:n relation because i just have to
save the task which has a projekt_id but with a m:n relations it is

thank you