how to make specific model field dynamic from another model - django

Question

I have two models Damaged and Product.In the Product model the quantity of the product depends upon the value of damaged_quantity which is stored in another table.For example if the damaged_quantity is in damaged table then the value of quantity in product should be quantity-damaged_quantity if the damaged.product_id == product.id .I tried like this but it is not working
models.py
class Damaged(models.Model):
product = models.ForeignKey('Product', on_delete=models.CASCADE)
damaged_quantity = models.IntegerField(default=0)
def __str__(self):
return self.product
class Product(models.Model):
name = models.CharField(max_length=250)
category = models.ForeignKey(Category,on_delete=models.CASCADE)
quantity = models.IntegerField()
views.py I access the queryset of product like this in my views
def list_products(request):
products = Product.objects.annotate(damaged_product_quantity=Sum('damagedproducts__damaged_quantity')).annotate(
real_quantity=ExpressionWrapper(F('quantity') - F('damaged_product_quantity'), output_field=IntegerField()))
list_product_template. Here while displaying the real_quantity if the damage.damaged_quanity and the product.quantity are euqal then it doesnot changes the value.Instead of becomming Zero it doesnot change the value.In other case it is working fine.
{% if not product.real_quantity %}
{{product.quantity}}
{% elif product.real_quantity == product.quantity %}
0
{% else %}
{{ product.real_quantity }}
{% endif %}
product_detail page
def product_detail(request, slug):
product = get_object_or_404(Product, slug=slug)
damaged = Damaged.objects.all()
return render(request, 'pos/product_detail.html', {'product': product,'damaged':damaged})
product_detail template.I tried like this to get the current quantity of products after adding damaged_products but it is nt working well.It is giving me both {% if %} and {% else %} part.How can i solve this?
product quantity:
{% for damage in damaged %}
{% if product.id == damage.product_id %}
{{product.quantity|subtract:damage.damaged_quantity}}
{% else %}
{{product.quantity}}
{% endif %}
{% endfor %}

Answer 1

I think you need to override the save method in Damaged model:
class Damaged(models.Model):
product = models.ForeignKey('Product', on_delete=models.CASCADE)
damaged_quantity = models.IntegerField(default=0)
def __str__(self):
return self.product
def save(self, *args, **kwargs):
super(Damaged, self).save(*args, **kwargs)
self.product.quantity = self.product.quantity - self.damaged_quantity
self.product.save()
But this solution might be inconsistent. For example, if you try to update the Damaged model, then value of the product will be updated again.
I would recommend using annotation to attach value with Product, so that you can query if needed. For example:
For this, lets add a related_name field in Damaged model:
class Damaged(models.Model):
product = models.ForeignKey('Product', on_delete=models.CASCADE, related_name='damagedproducts')
damaged_quantity = models.IntegerField(default=0)
Usage:
from django.db.models import Sum, F, IntegerField
products = Product.objects.annotate(damaged_product_quantity=Sum('damagedproducts__damaged_quantity')).annotate(real_quantity=ExpressionWrapper(F('quantity') - F('damaged_product_quantity'), output_field=IntegerField())
real_quantity_more_than_ten = products.filter(real_quantity__gt=10)
for p in real_quantity_more_than_ten:
print(p.real_quantity)
Update
from django.db.models import Sum
def product_detail(request, slug):
product = get_object_or_404(Product, slug=slug)
damaged = product.productdamaged.all()
if damaged.exists():
damage_amount = damaged.aggregate(d_amount = Sum('productdamaged__damaged_quantity')).get('d_amount', 0)
else:
damage_amount = 0
return render(request, 'pos/product_detail.html', {'product': product,'damage_amount':damage_amount})
# template
{% if damage_amount != 0 %}
{{product.quantity|subtract:damage_amount}}
{% else %}
{{product.quantity}}
{% endif %}