I've two tables brand and a product. each brand has multiple products.
So. I used prefetch_related to get related products for a particular brand with only a minimum product price. but the problem is when I have 2 products with the same price it selects both records so how to limit this?
alternatives_data = Brand.objects.filter(category__category_slug = category_slug).prefetch_related(
Prefetch('products', queryset=Product.objects.annotate(
min_brand_price=Min('brand__products__product_price')
).filter(
product_price=F('min_brand_price')
).order_by('product_id')))
i tried everything but nothing work!
To prevent a query to return multiple records with duplicata in specific columns, use the distinct method.
In your case, add .distinct('price') to the Product queryset inside the prefetch.
There is however one caveat : It is supported on PostgreSQL only.
Documentation
Related
I have three models named Category, Account, and AccountCategory. I need to be able to generate a report wherein I can show the number of which each account was categorized. (e.g let's say I have 10 accounts, then I have three categories (A, B, C) I need to be able to show a piechart)
Account has a many-to-many relationship with Category and AccountCategory is the junction table.
Ideally, I need to have a result of
Name
Slug
Percentage
Num of Accounts
A
a
40%
4
B
b
10%
1
C
c
50%
5
I was able to get the raw Query but I still need to get the total number of accounts so I can get the percentage I'm struggling with how to do this on ORM.
Basically, I did query the categories, joined account categories, and did a distinct on the account so it won't return the duplicates and then just total the result. For the ORM I think I need to filter the account categories to only return the latest account category per account and then total it but I can't seem to write the exact query using ORM I tried using the Subquery and Prefetch but no avail.
you can use annotate method in django orm for this
example :
from django.db.models.functions import Concat
from django.db.models import Value
queryset = YourModel.objects.filter().annotate(percentage = Concat('count',Value('%'))).values_list('name','slug','numofaccounts', 'percentage')
How to write the most efficient Django ORM query for the following scenario?
I need to get items based on a list of accountIds, but it will return duplicate records with the same accountId because accountId is not the primary key. Then I will need to remove the duplicates by only returning the last created record in the queryset.
I can use a for loop to loop through the list of accountIds and filter by each accountId and then order by the created date and get the latest one. However, with this approach, I will be calling the database so many times. There are more than 200 account Ids.
Are there better ways of doing this?
This could be useful
Model.objects.order_by('date_created').distinct()
docs: distinct in django queryset
if you are using postgres, it would be much useful and efficient
If using PostgreSQL you can add a field name to distinct() to create a SELECT DISTINCT ON (foo) query that returns the first unique value for that field. In your case if you order by account_id and then descending created_date you will get a single row per account_id that has the latest created_date
Item.objects.filter(
account_id__in=account_ids
).order_by(
'account_id', '-created_date'
).distinct('account_id')
Been searching the web for a couple hours now looking for a solution but nothing quite fits what I am looking for.
I have one model (simplified):
class SimpleModel(Model):
name = CharField('Name', unique=True)
date = DateField()
amount = FloatField()
I have two dates; date_one and date_two.
I would like a single queryset with a row for each name in the Model, with each row showing:
{'name': name, 'date_one': date_one, 'date_two': date_two, 'amount_one': amount_one, 'amount_two': amount_two, 'change': amount_two - amount_one}
Reason being I would like to be able to find the rank of amount_one, amount_two, and change, using sort or filters on that single queryset.
I know I could create a list of dictionaries from two separate querysets then sort on that and get the ranks from the index values ...
but perhaps nievely I feel like there should be a DB solution using one queryset that would be faster.
union seemed promising but you cannot perform some simple operations like filter after that
I think I could perhaps split name into its own Model and generate queryset with related fields, but I'd prefer not to change the schema at this stage. Also, I only have access to sqlite.
appreciate any help!
Your current model forces you to have ONE name associated with ONE date and ONE amount. Because name is unique=True, you literally cannot have two dates associated with the same name
So if you want to be able to have several dates/amounts associated with a name, there are several ways to proceed
Idea 1: If there will only be 2 dates and 2 amounts, simply add a second date field and a second amount field
Idea 2: If there can be an infinite number of days and amounts, you'll have to change your model to reflect it, by having :
A model for your names
A model for your days and amounts, with a foreign key to your names
Idea 3: You could keep the same model and simply remove the unique constraint, but that's a recipe for mistakes
Based on your choice, you'll then have several ways of querying what you need. It depends on your final model structure. The best way to go would be to create custom model methods that query the 2 dates/amount, format an array and return it
I'm new to django and ORM in general, and so have trouble coming up with query which would join multiple tables.
I have 4 Models that need joining - Category, SubCategory, Product and Packaging, example values would be:
Category: 'male'
SubCategory: 'shoes'
Product: 'nikeXYZ'
Packaging: 'size_36: 1'
Each of the Model have FK to the model above (ie. SubCategory has field category etc).
My question is - how can I filter Product given a Category (e.g. male) and only show products which have Packaging attribute available set to True? Obviously I want to minimise the hits on my database (ideally do it with 1 SQL query).
I could do something along these lines:
available = Product.objects.filter(packaging__available=True)
subcategories = SubCategory.objects.filter(category_id=<id_of_male>)
products = available.filter(subcategory_id__in=subcategories)
but then that requires 2 hits on database at least (available, subcategories) I think. Is there a way to do it in one go?
try this:
lookup = {'packaging_available': True, 'subcategory__category_id__in': ['ids of males']}
product_objs = Product.objects.filter(**lookup)
Try to read:
this
You can query with _set, multi __ (to link models by FK) or create list ids
I think this should work but it's not tested:
Product.objects.filter(packaging__available=True,subcategories__category_id__in=[id_of_male])
it isn't tested but I think that subcategories should be plural (related_name), if you didn't set related_name, then subcategory__set instead od subcategories should work.
Probably subcategories__category_id__in=[id_of_male] can be switched to .._id=id_of_male.
Consider the following model:
class Data(Model):
created_at = models.DateTimeField()
category = models.CharField(max_length=7)
I want to select the latest object for all categories.
Following this question, i'm selecting the distinct categories and then making a separate query for each of them:
categories = Data.objects.distinct('category').values_list('category', flat=True)
for category in categories:
latest_obj = Data.objects.filter(category=category).latest('created_at')
The downside of the approach is that it makes lots of queries (1 for the distinct categories, and then a separate query per category).
Is there a way to do this with a single query?
Typically, you would use a group by in relational database. Django has an aggergation API
(https://docs.djangoproject.com/en/dev/topics/db/aggregation/#aggregation) which allows you to do the following:
from django.db.models import Max
Data.objects.values('category').annotate(latest=Max('created_at'))
This will perform a single query and return a list like this:
[{'category' : 'cat1', 'latest' : '01/01/01' },{'category' : 'cat2' 'latest' : '02/02/02' }]
But I guess you might want to retrieve the data record id as well within this list. Django does not make thinks simple for you in this case. The problem is django uses all fields in the value clause to make the grouping and you cannot return extra columns from the query.
EDIT: I originally proposed to add a second values() clause to the end of the query based on web resources but this does not add extra columns to the result set.