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)
Related
Say I have peewee models like so:
class Users(_BaseModel):
id = AutoField(primary_key=True, null=False, unique=True)
first_name = CharField(null=False)
last_name = CharField(null=False)
# Cut short for clarity
class Cohorts(_BaseModel):
id = AutoField(primary_key=True, null=False, unique=True)
name = CharField(null=False, unique=True)
# Cut short for clarity
class CohortsUsers(_BaseModel):
cohort = ForeignKeyField(Cohorts)
user = ForeignKeyField(Users)
is_primary = BooleanField(default=True)
I need to access easily from the user what cohort they are in and for example the cohort's name.
If a user could be in just one cohort, it would be easy but here, having it be many2many complicates things.
Here's what I got so far, which is pretty ugly and inefficient
Users.select(Users, CohortsUsers).join(CohortsUsers).where(Users.id == 1)[0].cohortsusers.cohort.name
Which will do what I require it to but I'd like to find a better way to do it.
Is there a way to have it so I can do Users.get_by_id(1).cohort.name ?
EDIT: I'm thinking about making methods to access them easily on my Users class but I am not really sure it's the best way of doing it nor how to go about it
If it do it like so, it's quite ugly because of the import inside the method to avoid circular imports
#property
def cohort(self):
from dst_datamodel.cohorts import CohortsUsers
return Users.select(Users, CohortsUsers).join(CohortsUsers).where(Users.id == self.id)[0].cohortsusers.cohort
But having this ugly method allows me to do Users.get_by_id(1).cohort easily
This is all covered in the documentation here: http://docs.peewee-orm.com/en/latest/peewee/relationships.html#implementing-many-to-many
You have a many-to-many relationship, where a user can be in zero, one or many cohorts, and a cohort may have zero, one or many users.
If there is some invariant where a user only has one cohort, then just do this:
# Get all cohorts for a given user id and print their name(s).
q = Cohort.select().join(CohortUsers).where(CohortUsers.user == some_user_id)
for cohort in q:
print(cohort.name)
More specific to your example:
#property
def cohort(self):
from dst_datamodel.cohorts import CohortsUsers
cohort = Cohort.select().join(CohortsUsers).where(CohortUsers.user == self.id).get()
return cohort.name
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 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.
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)
I am trying to get a field in a queryset which depends on the result of the annotations. I tried using extra on an annotated queryset, which doesn't seem to work.
Lets say my models looks like this:
class Foo(models.Model):
name = models.CharField(...)
class FooData(models.Model):
foo = models.ForeignKey(Foo)
sales = models.PositiveIntegerField()
items_sold = models.PositiveIntegerField()
I have a queryset of Foo objects
some_foos = Foo.objects.filter()
I have annotated it to get the sum on FooData.
some_foos.object.annotate(
total_sales=Sum("foodata__sales"),
total_items_sold=Sum("foodata__items_sold")
)
Now I want to get the average price for which I am trying to use .extra. They don't seem to work. (I have looked the generated sql queries).
#Wont work, fields not defined yet.
some_foos.object.annotate(
total_sales=Sum("foodata__sales"),
total_items_sold=Sum("foodata__items_sold")
).extra(select={'price_avg': "total_sales/total_items_sold"})
#Wont work, this adds an extra clause in group by
price_avg = "SUM(appname_foodata.sales)/SUM(appname_foodata.items_sold)"
some_foos.object.annotate(
total_sales=Sum("foodata__sales"),
total_items_sold=Sum("foodata__items_sold")
)