'Return the user\'s bookmarks cache for a particular course.
Arguments:
course_key (CourseKey): course_key of the course whose bookmarks cache should be returned.
fetch (Bool): if the bookmarks should be fetched and cached if they already aren\'t.'

def _bookmarks_cache(self, course_key, fetch=False):

    store = modulestore()
    course_key = store.fill_in_run(course_key)
    if (course_key.run is None):
        return []
    cache_key = CACHE_KEY_TEMPLATE.format(self._user.id, course_key)
    bookmarks_cache = RequestCache.get_request_cache().data.get(cache_key, None)
    if ((bookmarks_cache is None) and (fetch is True)):
        bookmarks_cache = api.get_bookmarks(self._user, course_key=course_key, fields=DEFAULT_FIELDS)
        RequestCache.get_request_cache().data[cache_key] = bookmarks_cache
    return bookmarks_cache
