In [102]: Episode.objects.order_by('-date_modified').filter(series__series_type='series').values_list('series_id', flat=True).distinct()
Out[102]: [71L, 181L, 181L, 411L, 411L, 411L, 411L, 411L, 411L, 411L, 411L, 411L, 411L, 411L, 411L, 411L, 411L, 401L, 401L, 401L, '...(remaining elements truncated)...']