code: function(languageSlug) {
              let result;
              if(languageSlug) {
                  result = query('select p.*, c.slug as category_slug from project as p' +
                      ' left join category as c on c.id=p.category_id' +
                      ' where p.source_language_id in (select id from source_language where slug=?)' +
                      ' order by p.sort asc', [languageSlug]);
              } else {
                  result = query('select * from project' +
                      ' group  by slug order by sort asc');
              }
              for(let project of result) {
                  // store the language slug for convenience
                  project.source_language_slug = languageSlug;
              }
              return result;
          }
docs: Returns a list of projects available in the given language.
  
  @param languageSlug {string} if left null an array of all unique projects will be returned
  @returns {[{}]} an array of projects