'Update the active index for the given course\'s branch to point to new_id
:param index_entry:
:param course_locator:
:param new_id:'

def _update_head(self, course_key, index_entry, branch, new_id):

    if (not isinstance(new_id, ObjectId)):
        raise TypeError('new_id    must    be    an    ObjectId,    but    is    {!r}'.format(new_id))
    index_entry['versions'][branch] = new_id
    self.update_course_index(course_key, index_entry)
