[Question] Django 2 - how to sum values based on filter and group by Year-Month

I have the following table:

2018-01-01      10

2018-01-15      20

2018-01-31      30

2018-02-01      10

2018-03-01      10

2018-03-20      20

I need to Sum() the values filtering it with 

1 - sum(values) where date < first day of the month

2 - sum(values) where date < last day of the month

Expected result(something like that):

[

 {'year-month': '2018-01'}, {'sum_before_month_day_one': 0}, {'sum_before_last_month_day': 60},

 {'year-month': '2018-02'}, {'sum_before_month_day_one': 60}, {'sum_before_last_month_day': 70},

 {'year-month': '2018-03'}, {'sum_before_month_day_one': 70}, {'sum_before_last_month_day': 100},

]

So far I manage to sum values and group by month:

In [12]: result = Sale.objects \

    ...:     .annotate(date=TruncMonth('event_date')) \

    ...:     .values('date') \

    ...:     .annotate(quantity=Sum('quantity')) \

    ...:     .values('date', 'quantity') \

    ...:     .order_by('date')

In [13]: for i in result: print(i)

{'date': datetime.date(2018, 1, 1), 'quantity': 60.0}

{'date': datetime.date(2018, 2, 1), 'quantity': 10.0}

{'date': datetime.date(2018, 3, 1), 'quantity': 30.0}

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