def get_rendering_cache_key(placeholder_name, contentitem):
    """
    Return a cache key for the content item output.

    .. seealso::

        The :func:`ContentItem.clear_cache() <fluent_contents.models.ContentItem.clear_cache>` function
        can be used to remove the cache keys of a retrieved object.
    """
    if not contentitem.pk:
        return None
    return 'contentitem.@{0}.{1}.{2}'.format(placeholder_name, contentitem.
        plugin.type_name, contentitem.pk)
