views.py

def category(request, model, tag):
    """
    Given a lowercase model name and tag, search for all models linked to
    the given tag. The model is obtained by looking through a dictionary of
    available models.
    """
    available_models = {'apps': App, 'data': Data, 'ideas': Idea}
    try:
        actual_model = available_models[model]
    except KeyError:
        # The model does not exist.
        context = {'results': None}
    else:
        results = actual_model.objects.filter(tag=tag)
        context = {'results': results}
    return render_response(request, 'category.html', context)