Function get_modules_by_code 

pub async fn get_modules_by_code(
    conn: &mut PgConnection,
    course_id: Option<Uuid>,
    course_codes: &[String],
) -> ModelResult<HashMap<String, Vec<ModuleToList>>>

The active modules on each of course_codes, which share its roster. course_id narrows them to one course, as it does for get_schedules.