Django: Fill the gaps in a view which is grouped by month - django

Question

My problem seems to be quite standard and I found many solutions, but none which works for a Django view.
I have timestamped items ("conversions") and I want to show the number of items per month. My view deliver this:
2020-05 3
2020-03 2
What I need is this:
2020-05 3
2020-04 0
2020-03 2
models.py:
class Conversion(models.Model):
timestamp = models.DateTimeField()
views.py (thanks to this help):
from django.db.models import Count
from django.db.models.functions import TruncMonth
def conversions_monthly(request):
conv = Conversion.objects.values(month=TruncMonth(
'timestamp')).annotate(count=Count('pk')).order_by('-month')
context = {'conversion': conv}
return render(request, 'dummy/conversions_monthly.html', context)
Template conversions_monthly.html:
{% extends "dummy/base.html" %}
{% block content %}
<table>
{% for c in conversion %}
<tr>
<td>{{ c.month |date:"Y-m" }}</td>
<td class="text-right">{{ c.count }}</td>
</tr>
{% endfor %}
</table>
{% endblock content %}
Data:
[
{
"model": "dummy.conversion",
"pk": 1,
"fields": {
"project": 1,
"timestamp": "2020-03-10T05:00:00Z"
}
},
{
"model": "dummy.conversion",
"pk": 2,
"fields": {
"project": 1,
"timestamp": "2020-03-12T17:00:00Z"
}
},
{
"model": "dummy.conversion",
"pk": 3,
"fields": {
"project": 1,
"timestamp": "2020-05-19T12:00:00Z"
}
},
{
"model": "dummy.conversion",
"pk": 4,
"fields": {
"project": 2,
"timestamp": "2020-05-20T16:10:03Z"
}
},
{
"model": "dummy.conversion",
"pk": 5,
"fields": {
"project": 1,
"timestamp": "2020-05-20T16:30:00Z"
}
}
]
I guess, I have to "aggregate" somehow the min and max date and need something like:
import datetime
from dateutil.relativedelta import relativedelta
result = []
max = date(2020, 5, 20)
min = date(2020, 3, 1)
current = min
while current <= max:
result.append(current)
current += relativedelta(months=1)
which delivers [datetime.date(2020, 3, 1), datetime.date(2020, 4, 1), datetime.date(2020, 5, 1)]
I have no idea how to get these pieces together – or is there a completely different approach for that in Django?

Answer 1

You are on the right path already, here is your code stitched together
# views.py
def index(request):
result = []
# `min` and `max` are keywords in python, so it is bad practice to use them as variable names
_max = datetime.date(2020, 5, 20)
_min = datetime.date(2020, 3, 1)
current = _min
while current <= _max:
result.append({"date": current, "count": 0})
current += relativedelta(months=1)
conversions = Conversion.objects.values(month=TruncMonth(
'timestamp')).annotate(count=Count('pk')).order_by('-month')
# the conditions for checking `i` being smaller than `length` protect you in case there are no conversions in your range
i = 0
length = len(result)
for conversion in conversions:
while i < length and \
not (
result[i]["date"].year == conversion["month"].year and
result[i]["date"].month == conversion["month"].month
):
i += 1
if i < length:
result[i]["count"] = conversion["count"]
context = {'conversion': result}
return render(request, 'dummy/conversions_monthly.html', context)
template
{% extends "dummy/base.html" %}
{% block content %}
<table>
{% for c in conversion %}
<tr>
<td>{{ c.date |date:"Y-m" }}</td>
<td class="text-right">{{ c.count }}</td>
</tr>
{% endfor %}
</table>
{% endblock content %}

Answer 2

Inspired by Nader Alexan I found the following solution which delivers the desired result. _min and _max are no longer fix, but calculated now.
from django.db.models import Count
from django.db.models.functions import TruncMonth
from django.db.models import Max, Min
def conversions_monthly(request):
conversions = Conversion.objects.values(month=TruncMonth('timestamp')).annotate(
count=Count('pk'))
result = []
_min = Conversion.objects.aggregate(Min('timestamp'))[
'timestamp__min'].date().replace(day=1)
_max = Conversion.objects.aggregate(Max('timestamp'))[
'timestamp__max'].date().replace(day=1)
current = _max.replace(day=1)
while current >= _min:
c = conversions.filter(month__year=current.year).filter(
month__month=current.month)
if c:
result.append({"date": current, "count": c[0]['count']})
else:
result.append({"date": current, "count": 0})
current -= relativedelta(months=1)
context = {'conversion': result}
return render(request, 'dummy/conversions_monthly.html', context)