I have a class in my models.py
class Inventory(models.Model):
date = models.DateField(("Date"), default=datetime.now)
product = models.ForeignKey(Product)
stock_in = models.IntegerField()
stock_out = models.IntegerField()
balance = models.IntegerField()
particulars = models.CharField(max_length=250)
Now I want to add some stocks in the balance. Using the stock_in values to add certain numbers to the balance of a specific product in the Inventory class. Using an UpdateView to it, so that I can just Update the stock_in field then adding that value to the balance.
I'm currently using this, I've tried couple of solution in the internet but to no avail.
#property
def total(self):
return self.stock_in + self.balance
There is no 'official' mechanism in Django to do this. Recently, some ideas of adding some official solution to the Django framework were discussed in this thread on the django-developers mailing list. It might serve as an inspiration for what solution is currently best for your case.
Your method works well for simple calculations. If the property gets more expensive to calculate, using #cached_property can help a bit if the value is used multiple times.
You can also rely on the database to compute these values by adding an annotation to the queryset. This requires defining a custom Manager:
class InventoryManager(models.Manager):
def get_queryset(self):
super().get_queryset().annotate(total=F('stock_in') + F('balance'))
class Inventory(models.Model):
date = models.DateField(("Date"), default=datetime.now)
product = models.ForeignKey(Product)
stock_in = models.IntegerField()
stock_out = models.IntegerField()
balance = models.IntegerField()
particulars = models.CharField(max_length=250)
objects = InventoryManager()
This will add a balance attribute to your Inventory model instances if they are retreived using the default manager.
The problem with this approach (like discussed in the linked django-developers thread) is what your expectations are when modals are changed locally.
For example, with the custom manager in place, if I were to change stock_in for a modal, the value of total would still be valid for the value of stock_in at the time of retrieving it from the database:
>> qs = Inventory.objects.filter(date__gte=date(2017, 12, 22))
>> inventory0 = qs[0]
>> print(inventory0.total, inventory0.stock_in, inventory.balance)
100, 50, 50
>> inventory.balance = 100
>> print(inventory0.total, inventory0.stock_in, inventory.balance)
100, 50, 100
Also, an model instance not fetched from the db at all wont have a total attribute:
>> inventory = Inventory(stock_in=20, balance=10)
>> inventory.total
AttributeError: 'Inventory' object has no attribute 'total'
Adding a __getattr__ method to your class might be a solution to this usecase, but will still result in incorrect answers with local changes.
Related
I have a model in my django app like below:
models.py
class Profit(models.Model):
client = models.ForeignKey(Client, null=True, on_delete=models.CASCADE)
month = models.CharField(max_length=100)
amount = models.IntegerField()
total_profit = models.IntegerField()
Now, what I want to do is that whenever a new instance/object is created for this class, the user puts the month and the amount of profit for that month, But I want that it also calculates the total profit the user got up till the current profit, by adding all the profits that was being added in the past.
For example.
if the user is adding the profit for month April, then it add all the values in the amount field of previously added objects of (March, February, January and so on..) and put it in the field total_profit. So that the user can see how much total_profit he got at each new entry.
My views.py where I am printing the list of profits is given below:
views.py
class ProfitListView(ListView):
model = Profit
template_name = 'client_management_system/profit_detail.html'
context_object_name = 'profits'
# pk=self.kwargs['pk'] is to get the client id/pk from URL
def get_queryset(self):
user = get_object_or_404(Client, pk=self.kwargs['pk'])
return Profit.objects.filter(client=user)
Client is the another model in my models.py to which the Profit class is connected via ForeignKey
I also don't exactly know how to use window functions inside this view.
As stated in the comments one should generally not store things in the database that can be calculated from other data. Since that leads to duplication and then makes it difficult to update data. Although if your data might not change and this is some financial data one might store it anyway for record keeping purposes.
Firstly month as a CharField is not a very suitable field of yours for your schema. As firstly they are not easily ordered, secondly it would be better for you to work with a DateTimeField instead:
class Profit(models.Model):
month = models.CharField(max_length=100) # Remove this
made_on = models.DateTimeField() # A `DateTimeField` is better suited
amount = models.IntegerField()
total_profit = models.IntegerField()
Next since you want to print all the Profit instances along with the total amount you should use a Window function [Django docs] which will be ordered by made_on and we will also use a frame just in case that the made_on is same for two entries:
from django.db.models import F, RowRange, Sum, Window
queryset = Profit.objects.annotate(
total_amount=Window(
expression=Sum('amount'),
order_by=F('made_on').asc(),
frame=RowRange(end=0)
)
)
for profit in queryset:
print(f"Date: {profit.made_on}, Amount: {profit.amount}, Total amount: {profit.total_amount}")
I have written a python script in my project. I want to update the value of a field.
Here are my modes
class News_Channel(models.Model):
name = models.TextField(blank=False)
info = models.TextField(blank=False)
image = models.FileField()
website = models.TextField()
total_star = models.PositiveIntegerField(default=0)
total_user = models.IntegerField()
class Meta:
ordering = ["-id"]
def __str__(self):
return self.name
class Count(models.Model):
userId = models.ForeignKey(User, on_delete=models.CASCADE)
channelId = models.ForeignKey(News_Channel, on_delete=models.CASCADE)
rate = models.PositiveIntegerField(default=0)
def __str__(self):
return self.channelId.name
class Meta:
ordering = ["-id"]
This is my python script:
from feed.models import Count, News_Channel
def run():
for i in range(1, 11):
news_channel = Count.objects.filter(channelId=i)
total_rate = 0
for rate in news_channel:
total_rate += rate.rate
print(total_rate)
object = News_Channel.objects.filter(id=i)
print(total_rate)
print("before",object[0].total_star,total_rate)
object[0].total_star = total_rate
print("after", object[0].total_star)
object.update()
After counting the total_rate from the Count table I want to update the total star value in News_Channel table. I am failing to do so and get the data before the update and after the update as zero. Although total_rate has value.
The problem
The reason why this fails is because here object is a QuerySet of News_Channels, yeah that QuerySet might contain exactly one News_Channel, but that is irrelevant.
If you then use object[0] you make a query to the database to fetch the first element and deserialize it into a News_Channel object. Then you set the total_star of that object, but you never save that object. You only call .update() on the entire queryset, resulting in another independent query.
You can fix this with:
objects = News_Channel.objects.filter(id=i)
object = objects[0]
object.total_star = total_rate
object.save()
Or given you do not need any validation, you can boost performance with:
News_Channel.objects.filter(id=i).update(total_star=total_rate)
Updating all News_Channels
If you want to update all News_Channels, you actually better use a Subquery here:
from django.db.models import OuterRef, Sum, Subquery
subq = Subquery(
Count.objects.filter(
channelId=OuterRef('id')
).annotate(
total_rate=Sum('rate')
).order_by('channelId').values('total_rate')[:1]
)
News_Channel.objects.update(total_star=subq)
The reason is that object in your case is a queryset, and after you attempt to update object[0], you don't store the results in the db, and don't refresh the queryset. To get it to work you should pass the field you want to update into the update method.
So, try this:
def run():
for i in range(1, 11):
news_channel = Count.objects.filter(channelId=i)
total_rate = 0
for rate in news_channel:
total_rate += rate.rate
print(total_rate)
object = News_Channel.objects.filter(id=i)
print(total_rate)
print("before",object[0].total_star,total_rate)
object.update(total_star=total_rate)
print("after", object[0].total_star)
News_Channel.total_star can be calculated by using aggregation
news_channel_obj.count_set.aggregate(total_star=Sum('rate'))['total_star']
You can then either use this in your script:
object.total_star = object.count_set.aggregate(total_star=Sum('rate'))['total_star']
Or if you do not need to cache this value because performance is not an issue, you can remove the total_star field and add it as a property on the News_Channel model
#property
def total_star(self):
return self.count_set.aggregate(total_star=Sum('rate'))['total_star']
say I have two models like so...
class Product(models.Model):
...
overall_rating = models.IntegerField()
...
class Review(models.Model):
...
product = models.ForeignKey(Product, related_name='review', on_delete=models.CASCADE)
rating = models.IntegerField()
...
I want to use the ratings from all of the child Review objects to build an average overall_rating for the parent Product.
Question: I'm wondering how I may be able to achieve something like this using Django signals?
I am a bit of a newbie to this part of Django; have never really understood the signals part before.
This overall_rating value needs to be stored in the database instead of calculated using a method since I plan on ordering the Product objects based on their overall_rating which is done on a DB level. The method may look something like this if I were to implement it (just for reference):
def overall_rating(self):
review_count=self.review.count()
if review_count >= 1:
ratings=self.review.all().values_list('rating',flat=True)
rating_sum = 0
for i in ratings:
rating_sum += int(i)
return rating_sum / review_count
else:
return 0
Thank you
You want to update your Product after each save of Review. So the best and fastest way would be using post save method. For example, after each saved product you can get all reviews and calculate overall rating and then save it to the Product.
#receiver(post_save, sender=Review, dispatch_uid="update_overall_rating")
def update_rating(sender, instance, **kwargs):
parent = instance.product
all_reviews = Review.objects.filter(product=parent)
parent.overall_rating = get_overall_rating(all_reviews)
I'm writing an action to set one field equal to the number of instances attached to it via a foreign key (see below)
Models.py
class competition(models):
competition_name = models.CharField(max_length = 50)
class people(models):
competition = models.ForeignKey(competition)
fullname = models.CharField(max_length = 100)
Admin.py
def people_in_competition:
for X in queryset:
X.number_of_people = X.count(X.set_all) #(I want the number of people in this competition in this part)
X.save()
Of course this gives me an error as I cant seem to use _set_all in admin.py, does it only work in templates? What would be the best way to figure that number out?
Use the backward relation:
X.number_of_people = X.people_set.all().count()
I've looked at doing a query using an extra and/or annotate but have not been able to get the result I want.
I want to get a list of Products, which has active licenses and also the total number of available licenses. An active license is defined as being not obsolete, in date, and the number of licenses less the number of assigned licenses (as defined by a count on the manytomany field).
The models I have defined are:
class Vendor(models.Model):
name = models.CharField(max_length=200)
url = models.URLField(blank=True)
class Product(models.Model):
name = models.CharField(max_length=200)
vendor = models.ForeignKey(Vendor)
product_url = models.URLField(blank=True)
is_obsolete = models.BooleanField(default=False, help_text="Is this product obsolete?")
class License(models.Model):
product = models.ForeignKey(Product)
num_licenses = models.IntegerField(default=1, help_text="The number of assignable licenses.")
licensee_name = models.CharField(max_length=200, blank=True)
license_key = models.TextField(blank=True)
license_startdate = models.DateField(default=date.today())
license_enddate = models.DateField(null=True, blank=True)
is_obsolete = models.BooleanField(default=False, help_text="Is this licenses obsolete?")
licensees = models.ManyToManyField(User, blank=True)
I have tried filtering by the License model. Which works, but I don't know how to then collate / GROUP BY / aggregate the returned data into a single queryset that is returned.
When trying to filter by procuct, I can quite figure out the query I need to do. I can get bits and pieces, and have tried using a .extra() select= query to return the number of available licenses (which is all I really need at this point) of which there will be multiple licenses associated with a product.
So, the ultimate answer I am after is, how can I retrieve a list of available products with the number of available licenses in Django. I'd rather not resort to using raw as much as possible.
An example queryset that gets all the License details I want, I just can't get the product:
License.objects.annotate(
used_licenses=Count('licensees')
).extra(
select={
'avail_licenses': 'licenses_license.num_licenses - (SELECT count(*) FROM licenses_license_licensees WHERE licenses_license_licensees.license_id = licenses_license.id)'
}
).filter(
is_obsolete=False,
num_licenses__gt=F('used_licenses')
).exclude(
license_enddate__lte=date.today()
)
Thank you in advance.
EDIT (2014-02-11):
I think I've solved it in possibly an ugly way. I didn't want to make too many DB calls if I can, so I get all the information using a License query, then filter it in Python and return it all from inside a manager class. Maybe an overuse of Dict and list. Anyway, it works, and I can expand it with additional info later on without a huge amount of risk or custom SQL. And it also uses some of the models parameters that I have defined in the model class.
class LicenseManager(models.Manager):
def get_available_products(self):
licenses = self.get_queryset().annotate(
used_licenses=Count('licensees')
).extra(
select={
'avail_licenses': 'licenses_license.num_licenses - (SELECT count(*) FROM licenses_license_licensees WHERE licenses_license_licensees.license_id = licenses_license.id)'
}
).filter(
is_obsolete=False,
num_licenses__gt=F('used_licenses')
).exclude(
license_enddate__lte=date.today()
).prefetch_related('product')
products = {}
for lic in licenses:
if lic.product not in products:
products[lic.product] = lic.product
products[lic.product].avail_licenses = lic.avail_licenses
else:
products[lic.product].avail_licenses += lic.avail_licenses
avail_products = []
for prod in products.values():
if prod.avail_licenses > 0:
avail_products.append(prod)
return avail_products
EDIT (2014-02-12):
Okay, this is the final solution I have decided to go with. Uses Python to filter the results. Reduces cache calls, and has a constant number of SQL queries.
The lesson here is that for something with many levels of filtering, it's best to get as much as needed, and filter in Python when returned.
class ProductManager(models.Manager):
def get_all_available(self, curruser):
"""
Gets all available Products that are available to the current user
"""
q = self.get_queryset().select_related().prefetch_related('license', 'license__licensees').filter(
is_obsolete=False,
license__is_obsolete=False
).exclude(
license__enddate__lte=date.today()
).distinct()
# return a curated list. Need further information first
products = []
for x in q:
x.avail_licenses = 0
x.user_assigned = False
# checks licenses. Does this on the model level as it's cached so as to save SQL queries
for y in x.license.all():
if not y.is_active:
break
x.avail_licenses += y.available_licenses
if curruser in y.licensees.all():
x.user_assigned = True
products.append(x)
return q
One strategy would be to get all the product ids from your License queryset:
productIDList = list(License.objects.filter(...).values_list(
'product_id', flat=True))
and then query the products using that list of ids:
Product.objects.filter(id__in=productIDList)