Negative Stock Prevention

[tech george]

Hello,

Please help crack the below code, I want to prevent negative stock, and if the stock is == 0, deduct it from reorder_level instead.

Currently, the stock goes negative.

models.py

class Stock(models.Model):
    quantity = models.IntegerField(default='0', blank=True, null=True)
    reorder_level = models.IntegerField(default='0', blank=True, null=True)

class Dispense(models.Model):
    drug_id = models.ForeignKey(Stock, on_delete=models.SET_NULL,null=True,blank=False)
    dispense_quantity = models.PositiveIntegerField(default='1', blank=False, null=True)
    taken=models.CharField(max_length=300,null=True, blank=True)

views.py

try:  
    
    if request.method == 'POST':
        if form.is_valid(): 
            username = form.cleaned_data['taken']
            qu=form.cleaned_data['dispense_quantity']
            ka=form.cleaned_data['drug_id']
            # print(username)
        
        
                
            stock= eo=Stock.objects.annotate(
            expired=ExpressionWrapper(Q(valid_to__lt=Now()), output_field=BooleanField())
            ).filter(expired=False).get(id=username)
            form=DispenseForm(request.POST or None, instance=stock)
            instance=form.save()
            # print(instance)
            if quantity > 0
                instance.quantity-=qu
                instance.save()
            else:
                instance.reorder_level-=qu
                instanc.save()

            form=DispenseForm(request.POST or None ,initial={'patient_id':queryset})
            form.save()

            messages.success(request, "Drug Has been Successfully Dispensed")

            return redirect('manage_patient_pharmacist')
        else:
            messages.error(request, "Validity Error")

            return redirect('manage_patient_pharmacist')

    context={
        "patients":queryset,
        "form":form,
        # "stocks":stock,
        "drugs":drugs,
        "prescrips":prescrips,
        "expired":ex,
        "expa":eo,

        }
    if request.method == 'POST':
    
        print(drugs)
        context={
            "drugs":drugs,
            form:form,
            "prescrips":prescrips,
            "patients":queryset,
            "expired":ex,
            "expa":eo,

        }
except:
    messages.error(request, "Dispensing Not Allowed! The Drug is Expired ,please do a re-stock ")
    return redirect('manage_patient_pharmacist')
context={
        "patients":queryset,
        "form":form,
        # "stocks":stock,
        "drugs":drugs,
        "prescrips":prescrips,
        "expired":ex,
        "expa":eo,

        }

return render(request,'templates/discharge.html',context)

[Derek]

As a start, the logic checks for the model should not be in views.py but rather with the model object (in models.py).  You can extend the save() method, for example, and add the checks there.

See: 

* https://docs.djangoproject.com/en/4.1/ref/models/instances/#saving-objects

* https://docs.djangoproject.com/en/4.1/ref/models/instances/#what-happens-when-you-save
* https://docs.djangoproject.com/en/4.1/topics/db/models/#overriding-model-methods

[Ryan Nowakowski]

On Mon, Aug 29, 2022 at 05:18:39PM +0300, tech george wrote:
> Please help crack the below code, I want to prevent negative stock, and if
> the stock is == 0, deduct it from reorder_level instead.
> Currently, the stock goes negative.
>
> models.py
>
> class Stock(models.Model):
> quantity = models.IntegerField(default='0', blank=True, null=True)
> reorder_level = models.IntegerField(default='0', blank=True, null=True)
>
> class Dispense(models.Model):
> drug_id = models.ForeignKey(Stock,
> on_delete=models.SET_NULL,null=True,blank=False)
> dispense_quantity = models.PositiveIntegerField(default='1',
> blank=False, null=True)
> taken=models.CharField(max_length=300,null=True, blank=True)

Maybe change quantity and reorder_level to PositiveIntegerField?

[Thomas Couch]

I don't see where you define the quantity variable, should that be instance.quantity? Also, presumably you want to check if quantity is greater than or equal to qu rather than 0.

Try changing `if quantity > 0` to `if instance.quantity >= qu`

[tech george]

Hello,

Sorry for the late reply.

I changed the models as below and added checkConstraint , But when I migrate I get the below error.

What am I still doing wrong?

class Stock(models.Model):
    quantity = models.PositiveIntegerField(default='0', blank=True, null=True)
    reorder_level = models.PositiveIntegerField(default='0', blank=True, null=True)

class Meta:
        CheckConstraint(check=Q(quantity__gt=0), name='quantity')

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[Ryan Nowakowski]

I don't see any error. Did you forget to post it?

[Ammar Mohammed]

I don't think you need that constraint after using the PositiveIntegerField .

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[tech george]

Hello,

Sorry the error is:

django.db.utils.IntegrityError: CHECK constraint failed: quantity

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[tech george]

I managed to remove the error,

I had a negative stock in my DB.

Thanks a lot for the help.

On that note, I was also trying to get the dispensed from stock

I was trying to do this but I don't think it can work since there is already a foreign key at Dispense model fetching drug_id from stock.

What's the best way out?

@property
def dispensed(self):
    return (self.quantity - self.dispense.quantity)

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[Ryan Nowakowski]

If you get this error during migration you likely have some existing Stock rows in your database with quantity that is less than 0. If this site is in production already, you'll need to create a data migration to change your negative quantities to zero.

https://docs.djangoproject.com/en/4.1/topics/migrations/#data-migrations

If this project isn't in production yet, just delete your database and migrations and start from ground zero.