query to django model to find best company sale in the month - django

Question

I have two django model one "company" and the other is "MonthlyReport" of the company
I want to find out which company sale in current month had more than 20% of previous month sale
class Company(models.Model):
name = models.CharField(max_length=50)
class MonthlyReport(models.Model):
company = models.ForeignKey(Company,on_delete=models.CASCADE)
sale = models.IntegerField()
date = models.DateField()
How can i figure out this issue to find a company that has more than 20% sales over the previous month

Answer 1

You can certainly do it using the ORM. You will need to combine Max (or SUM depending on your use case) with a Q() expression filter and annotate the percentage increase to the queryset before filtering it.
You could do it in a single piece of code, but I have split it out because getting the dates and the query expressions are quite long. I have also put the increase value in a separate variable, rather than hardcoding it.
from datetime import datetime, timedelta
from django.db.models import Max, Q
SALES_INCREASE = 1.2
# Get the start dates of this month and last month
this_month = datetime.now().date().replace(day=1)
last_month = (this_month - timedelta(days=15)).replace(day=1)
# Get the maximum sale this month
amount_this_month = Max('monthlyreport__sale',
filter=Q(monthlyreport__date__gte=this_month))
# Get the maximum sale last month, but before this month
amount_last_month = Max('monthlyreport__sale',
filter=Q(monthlyreport__date__gte=last_month) & \
Q(monthlyreport__date__lt=this_month))
Company.objects.annotate(
percentage_increase=amount_this_month/amount_last_month
).filter(percentage_increase__gte=SALES_INCREASE)
Edit - removed incorrect code addition

Answer 2

There is probably a way to do this using ORM, but I would just go with python way:
First add related name to MonthlyReport
class Company(models.Model):
name = models.CharField(max_length=50)
class MonthlyReport(models.Model):
company = models.ForeignKey(Company, related_name="monthly_reports", on_delete=models.CASCADE)
sale = models.IntegerField()
date = models.DateField()
Then
best_companies = []
companies = Company.objects.all()
for company in companies:
two_last_monthly_reports = company.monthly_reports.order_by("date")[:2]
previous_report = two_last_monthly_reports[0]
current_report = two_last_monthly_reports[1]
if current_report.sale / previous_report.sale > 1.2:
best_companies.append(company)