title: Django queryset to a string
body: <p>I have a queryset by doing:</p>
  
  <pre><code>chart_data = DetailChange.objects.extra(select={'day': 'date(captured_date)'})\
      .values('item_id__store_id__store_name', 'day')\
      .annotate(total_sale=Sum('amount_sold'))
  </code></pre>
  
  <p>then I have a queryset like this:</p>
  
  <pre><code>{'day': datetime.date(2019, 6, 24), 'item_id__store_id__store_name': 'Keyboard Pro', 'total_sale': 706.0}
  {'day': datetime.date(2019, 6, 25), 'item_id__store_id__store_name': 'Keyboard Pro', 'total_sale': 18.0}
  ...
  </code></pre>
  
  <p>now what I want to do is get one single string that combines all the days in it, separated by ",". Like:</p>
  
  <pre><code>"2019-6-24, 2019-6-25, 2019-6-26, ..."
  </code></pre>
  
  <p>Is there an easy way to get it done?</p>