Develop Reference

what is the best way to use prefetch_related with multitable inheritance and multiple types of discounts for an ecommerce website

i have the following models:
class Theme(models.Model):
name = mode.charfield()
class Category(mdoesl):
name = models.charfield()
class Product(models.Model):
title = models.charfield()
class Course(Product):
..........
class LtProduct(Product):
category = models.ForeignKey(Category)
theme = models.ForeginKey(Theme)
class CouponCode(models.Model):
code = models.Charfield(unique = True)
.........
class ProductDiscount(models.Model):
product = models.ForeignKey(Product)
codes = models.ManyToManyField(CouponCode)
class CategoryDiscount(models.Model):
category = models.ForeignKey(Category)
class ThemeDiscount(models.Model):
theme = models.ForeignKey(Theme)
class UserCoupon(models.Model):
user = models.OneToOneField(User)
code = models.ForeignKey(CouonCode)
is_used = models.BooleanField(default = False)
what I want to do here is first to get the user code and check if that code can be applied to add this product to the user cart, and also fetch the category and theme related to this product and with their discounts and codes too, here's what I was trying to solve this
product = Product.objects.prefetch_related('course', 'lvproduct__category__categorydiscount_set__codes', 'lvproduct__theme__theme_discount_set__codes','productdiscount_set__codes').get(id = product_id)
and then I would fetch the user redeemed code by this:
user_code = UserCoupon.objects.filter(user = user, is_used = False)
and then I check by each type of discount to check if the codes belong to these discounts
codes = product.productdiscount_set.codes.all()
for code in codes:
if code == user_code:
#do the necessary calculation
user_code.is_used = True
if the code is not in the product discount codes then I will check if it's for the category but since the category is only related to LvProduct I would do this:
if not user_code.is_used:
try:
codes = product.lvproduct.category.categorydiscount_set.codes.all()
#do the same thing here
and since the same coupon code can be used for multiple types of discounts, and if two discounts are applicable for the same product the first discount type that will be checked first is the one that will be applied, so is this the best way to achieve this?
I hope this is clear enough of what I am trying to do

How to reduce quantity of an item in main table when it is being used in another table - django

I am creating my model in Django and I have a many to many relationship between supplies and van kits. The idea is that an "item" can belong to many "van kits" and a "van kit" can have many " items. I created an intermediary model that will hold the relationship, but I am struggling to figure out a way to relate the quantity in the van kit table to the quantity in the main supplies table. For example, if I wanted to mark an item in the van kit as damaged and reduce the quantity of that supply in the van kit, I would also want to reduce the total count of that supply in the main "supplies" table until it has been replenished. I am thinking that maybe I'll have to create a function in my views file to carry out that logic, but I wanted to know if it could be implemented in my model design instead to minimize chances of error. Here's my code:
class supplies(models.Model):
class Meta:
verbose_name_plural = "supplies"
# limit the user to selecting a pre-set category
choices = (
('CREW-GEAR','CREW-GEAR'),
('CONSUMABLE','CONSUMABLE'),
('BACK-COUNTRY','BACK-COUNTRY')
)
supplyName = models.CharField(max_length=30, blank=False) # if they go over the max length, we'll get a 500 error
category = models.CharField(max_length=20, choices = choices, blank=False)
quantity = models.PositiveSmallIntegerField(blank=False) # set up default
price = models.DecimalField(max_digits=5, decimal_places=2, null=True, blank=True) # inputting price is optional
def __str__(self):
return self.supplyName
class van_kit(models.Model):
supply_name = models.ManyToManyField(supplies, through='KitSupplies',through_fields=('vanKit','supplyName'), related_name="supplies")
van_kit_name = models.CharField(max_length=100)
vanName = models.ForeignKey(vans, on_delete=models.CASCADE)
def __str__(self):
return self.van_kit_name
class KitSupplies(models.Model):
supplyName = models.ForeignKey(supplies, on_delete=models.CASCADE)
vanKit = models.ForeignKey(van_kit, on_delete=models.CASCADE)
quantity = models.PositiveSmallIntegerField(blank=False)
def __str__(self):
return str(self.supplyName)
class Meta:
verbose_name_plural = 'Kit Supplies'
I am fairly new to django, I have to learn it for a class project so if my logic is flawed or if a better way to do it is obvious, please respectfully let me know. I'm open to new ways of doing it. Also, I've read through the documentation on using "through" and "through_fields" to work with the junction table, but I'm worried I may not be using it correctly. Thanks in advance.

One option would be to drop/remove the field quantity from your supplies model and just use a query to get the total quantity.
This would be a bit more expensive, as the query would need to be run each time you want to know the number, but on the other hand it simplifies your design as you don't need any update logic for the field supplies.quantity.
The query could look as simple as this:
>>> from django.db.models import Sum
>>> supplies_instance.kitsupplies_set.aggregate(Sum('quantity'))
{'quantity__sum': 1234}
You could even make it a property on the model for easy access:
class supplies(models.Model):
...
#property
def quantity(self):
data = self.kitsupplies_set.aggregate(Sum('quantity'))
return data['quantity__sum']

Django-Tables2 add extra columns from dictionary

I apologize if this question has been asked before but I couldn't find my specific use case answered.
I have a table that displays basic product information. Product details such as price, number of sales, and number of sellers are scraped periodically and stored in a separate database table. Now I want to display both the basic product information and scraped details in one table on the frontend using tables2. To do this, I wrote a function in my Product model to fetch the latest details and return them as a dictionary this way I can use a single Accessor call.
# models.py
class Product(models.Model):
created_at = models.DateTimeField(auto_now_add=True)
name = models.CharField(max_length=256)
brand = models.ForeignKey(Brand)
category = models.CharField(max_length=128, choices=CATEGORY_CHOICES)
def __unicode__(self):
return self.name
def currentState(self):
currentDetailState = ProductDetailsState.objects.filter(
product=self
).latest('created_at')
# return current details as a dictionary
return {
price: currentDetailState.price,
num_sellers: currentDetailState.num_sellers,
num_sales: currentDetailState.num_sales
}
class ProductDetailsState(models.Model):
product = models.ForeignKey(Product)
created_at = models.DateTimeField(auto_now_add=True)
price = models.DecimalField(max_digits=6, decimal_places=2, null=True)
num_sellers = models.IntegerField(null=True)
num_sales = models.IntegerField(null=True)
def __unicode__(self):
return self.created_at
# tables.py
class ProductTable(tables.Table):
productBrand = tables.Column(
accessor=Accessor('brand.name'),
verbose_name='Brand'
)
currentRank = tables.Column(
accessor=Accessor('currentRank')
)
class Meta:
model = Product
...
How do I now use this returned dictionary and split it into columns in my Product table? Is there another way to use an Accessor than how I am doing it?

You can use the Accessor to traverse into the dict, so something like this should work:
class ProductTable(tables.Table):
# brand is the name of the model field, if you use that as the column name,
# and you have the __unicode__ you have now, the __unicode__ will get called,
# so you can get away with jus this:
brand = tables.Column(verbose_name='Brand')
currentRank = tables.Column()
# ordering on the value of a dict key is not possible, so better to disable it.
price = tables.Column(accessor=tables.A('currentState.price'), orderable=False)
num_sellers = tables.Column(accessor=tables.A('currentState.num_sellers'), orderable=False)
num_sales = tables.Column(accessor=tables.A('currentState.num_sales'), orderable=False)
class Meta:
model = Product
While this works, sorting is also nice to have. In order to do that, your 'currentState' method is a bit in the way, you should change the QuerySet you pass to the table. This view shows how that could work:
from django.db.models import F, Max
from django.shortcuts import render
from django_tables2 import RequestConfig
from .models import Product, ProductDetailsState
from .tables import ProductTable
def table(request):
# first, we make a list of the post recent ProductDetailState instances
# for each Product.
# This assumes the id's increase with the values of created_at,
# which probably is a fair assumption in most cases.
# If not, this query should be altered a bit.
current_state_ids = Product.objects.annotate(current_id=Max('productdetailsstate__id')) \
.values_list('current_id', flat=True)
data = Product.objects.filter(productdetailsstate__pk__in=current_state_ids)
# add annotations to make the table definition cleaner.
data = data.annotate(
price=F('productdetailsstate__price'),
num_sellers=F('productdetailsstate__num_sellers'),
num_sales=F('productdetailsstate__num_sales')
)
table = ProductTable(data)
RequestConfig(request).configure(table)
return render(request, 'table.html', {'table': table})
This simplifies the table definition, using the annotations created above:
class ProductTable(tables.Table):
brand = tables.Column(verbose_name='Brand')
currentRank = tables.Column()
price = tables.Column()
num_sellers = tables.Column()
num_sales = tables.Column()
class Meta:
model = Product
You can find the complete working django project at github

Calculate a number in a Django model from another model's data

I want to take data (amount_spent) from the field of each user and add those numbers up and display them in another field (total_revenue) from a different model (RevenueInfo).
from __future__ import unicode_literals
from django.contrib.auth.models import User
from django.db import models
from django import forms, views
# Create your models here.
#LoginInfo is being used, LoginForms in forms.py is
class LoginInfo(models.Model):
username = models.CharField('', max_length=10)
password = models.CharField('', max_length=15)
class ExtendedProfile(models.Model):
user = models.OneToOneField(User)
amount_spent = models.DecimalField(max_digits=6, decimal_places=2)
class RevenueInfo(models.Model):
total_amount_spent = models.DecimalField(max_digits=6, decimal_places=2, default=0)
total_revenue = models.ForeignKey(ExtendedProfile, null=True)
class Product(models.Model):
category = models.CharField(max_length=100)
name = models.CharField(max_length=100)
description = models.TextField()
#photo = models.ImageField()
price_CAD = models.DecimalField(max_digits=6, decimal_places=2)
quantity = models.DecimalField(max_digits=2, decimal_places=0, null=True)
How could I go about this? Would I iterate of each Usermodel and find User.amount_spent then add that to RevenueInfo.total_revenue? I'm not sure how to put that into code. Also I'm pretty sure I don't need both total_amount_spent and total_revenue but I feel like I need a ForeignKey

You could add a classmethod to the ExtendedProfile model to aggregate the amount_spent value for each User (which bypasses the need for a separate RevenueInfo model):
from django.db.models import Sum
class ExtendedProfile(models.Model):
....
#classmethod
def total_user_spend(cls):
return cls.objects.aggregate(total=Sum('amount_spent'))
Then you can get the total spend using ExtendedProfile.total_user_spend():
>>> ExtendedProfile.total_user_spend()
{'total': Decimal('1234.00')}

Yes, you can write a method for that in your model. There are 2 ways for it.
1) Writing a method that calculates the values and sets it to a instance value.
2) Writing a method that calculates the value and directly returns it.
For example purpose, here is the code for 2nd type.
# models.py
def total_amount_spent(self):
temp_values = [int(user.amount_spent) for user in ExtendedProfile.objects.all()]
return sum(temp_values)
And for using that value in views , but remeber it would be an integer by default
#views.py
value = RevenueInfo.total_amount_spent()

Avoid iterating over database entities in python (it can get really slow). Look into aggregation, it allows you to efficiently get sum (average, max, min, etc...) of values in a database:
>>> from django.db.models import Sum
>>> ExtendedProfile.objects.all().aggregate(Sum('amount_spent'))
{'amount_spent__sum': Decimal('1234.56')}
>>> # ... do whatever you want with the return value

Dynamic field value in Django class

I want to create one dynamic field value for my class in Django using PyCharm.
CATEGORY_CHOICES = (
('on','one'),
('tw','two'),
('th','three'),
('fo','four'),
('fi','five'),
)
class art(models.Model):
Title=models.CharField(max_length=300)
Desciption=models.TextField()
Category=models.CharField(max_length=2, choices=CATEGORY_CHOICES)
I want the category field in my class to take more than one option, maybe two or more.
Any help would be appreciated.

If you want one python model to have multiple categories, then you need django ManyToManyField. Basically one model object could have multiple choices, one choice can also belong to multiple models objects:
class Category(models.Model):
category_name = models.CharField(max_length=10, unique=True)
class Art(models.Model):
title = models.CharField(max_length=300)
description = models.TextField()
category = models.ManyToManyField('Category', blank=True)
Note that I put unique=True for category_name to avoid creating duplicate categories.
Something not related, you shouldn't use lower fist in model name, and upper first for field name, that's really BAD naming convention and might confuse others who read your code.
Example:
# create your category in code or admin
one = Category.objects.create(category_name='one')
two = Category.objects.create(category_name='two')
three = Category.objects.create(category_name='three')
# create a new art obj
new_art = Art.objects.create(title='foo', description='bar')
# add category to Art obj
new_art.category.add(one)
new_art.category.add(two)
# category for new art obj
new_art_category = new_art.category.all()
# get only a list of category names
category_names = new_art_category.values_list('category_name', flat=True)
# create another Art obj
new_art2 = Art.objects.create(title="test", description="test")
# assign category to new_art2
new_art2.category.add(two)
new_art2.category.add(three)
Django doc for many to many and python pep8 doc.