def back_feed(who_id, whom_id):
    """Takes 5 lastest posts from user with ``who_id`` places them in user
    with ``whom_id`` feed.

    The reason behind this is that new users may follow someone but still have
    and empty feed, which makes them sad :( so we'll give them some. If the
    posts are to old for a non user they will be removed when the feed is
    trimmed, but they may make it in to the feed but not at the top.

    :param who_id: user who just followed ``who_id``
    :type who_id: str
    :param whom_id: user who was just followed by ``whom_id``
    :type whom_id: str
    :returns: None

    """
    # Get followee's last 5 un-approved posts (doesn't matter if isn't any)
    # We only need the IDs and the created time
    posts = m.db.posts.find(
        {'user_id': whom_id, 'reply_to': None,
         'permission': {'$lte': k.PERM_PJUU}},
        {'_id': True, 'created': True},
    ).sort('created', -1).limit(5)

    # Iterate the cursor and append the posts to the users feed
    for post in posts:
        timestamp = post.get('created')
        post_id = post.get('_id')
        # Place on the feed
        r.zadd(k.USER_FEED.format(who_id), {str(post_id): timestamp})
        # Trim the feed to the 1000 max
        r.zremrangebyrank(k.USER_FEED.format(who_id), 0, -1000)