Django Help Needed urgently

[mahmood]

I am trying to create a simple inventory app using Django. My project came to a stand still when I couldn't find a solution for calculating opening and closing balances on the fly/dynamically. I tried for days researching on the web but no progress yet. If anyone could help i will appreciate.

All I need is a query to calculate opening and closing balances dynamically.

My Models:

class Item(models.Model):
name = models.CharField(max_length=30)
buying_price = models.PositiveIntegerField()
selling_price = models.PositiveIntegerField()
quantity = models.PositiveIntegerField()

def __str__(self):
    return self.name    

class SalesSheet(models.Model):
txn_date = models.DateField()
item = models.ForeignKey(Item, on_delete=models.CASCADE)
purchases = models.PositiveIntegerField()
sales = models.PositiveIntegerField()

What I want after posting purchases and sales is an output like this

Output:
id  date        item  opening  purchases sales closing
 1  1-1-19      abc   10       20        5     25
 2  2-12-19     def   25       20        10    35
 3  3-1-19      abc   25       10        25    10
 4  4-1-19      def   35       10        30    15
 5  7-1-19      abc   10       0         0     10
 6  9-1-19      def   15       0         5     10

and so on.....

I am using item.quantity to store opening stock for the first time only.

Take into consideration that there will be different items and also back dated entries.

help me guys.