@Ralf
TODO: migration /Users/groucho/Documents/HPI/workspace/Xikolo-CodeOcean/codeocean/db/migrate/20160204111716_add_user_to_code_harbor_link.rb
is broken. A reference is made to table users which does not exist.
Workaround: run rake db:schema:load before running db:migrate. Still this should be fixed.