How do you choose a media item at random from a Page Connection in a Zotonic template?

Question

I would like to render a random connected media item in Zotonic as follows:

{% with m.rsc[id].banner as media %} 
    {% if media %}
    {% with media.random as m %}
            {% media m %}
        {% endwith %}
{% endif %} 
{% endwith %}

How do you choose a media item at random from a Page Connection in a Zotonic template?

Answer 1

Arjan has a partial answer here: http://groups.google.com/group/zotonic-users/msg/021ac08702176745

However, the random filter does not appear to work with my resource collections.

Ideally, using Arjan's solution, the template would be the beautifully concise:

{% with m.rsc[id].banner|random as m %} 
    {% if m %}
        {% media m %}
    {% endif %} 
{% endwith %}

This looks like a bug in the random filter, so I am going to debug it and post back here.

Answer 2

Not all filters take all kinds of semi-lists. (ie. data structures that can act like a list but aren't, search results and resource lists are examples.)

You can force a value to be a list and then apply the random filter:

{% media m.rsc[id].banner|make_list|random %}

Also makes use of the fact that {% media %} on an undefined value returns the empty list.