I have the following 3 models related by Foreign Key as following:
class Seller(models.Model):
name = models.CharField(max_length=20)
def __str__(self):
return self.name
class Genre(models.Model):
seller= models.ForeignKey(Seller, related_name="genre", on_delete=models.CASCADE)
name = models.CharField(max_length=20)
def __str__(self):
return self.name
class Book(models.Model):
genre= models.ForeignKey(Genre, related_name="book", on_delete=models.CASCADE)
name = models.CharField(max_length=20)
def __str__(self):
return self.name
And I want to retrieve the whole 3 tables in one database query, by querying the Seller objects, as following:
sellers = Seller.objects.select_related('genre', 'book').all().values('name')
seller_df = pd.DataFrame(list(sellers))
What is the syntax to filter for books carried by a particular seller, without hitting the database again (by utilizing either the Seller queryset or the pandas seller_df)
seller1 = seller_df ['name'].iloc[0]
seller1_books = Book.objects.filter(...)
seller_last = seller_df ['name'].iloc[-1]
seller_last_books = Book.objects.filter(...)
I dont know so mach about caching but I know something that you like:
We use select_related when the object is single like onetoone or fk.
.
for many to many or reverse fk like your example use prefetch_related
Related
I have three models as following, with Seller as the grandparent, Genre as the parent and Book as the chidlren:
class Seller(models.Model):
name = models.CharField(max_length=20)
def __str__(self):
return self.name
class Genre(models.Model):
seller= models.ForeignKey(Seller, related_name="genre", on_delete=models.CASCADE)
name = models.CharField(max_length=20)
def __str__(self):
return self.name
class Book(models.Model):
genre= models.ForeignKey(Genre, related_name="book", on_delete=models.CASCADE)
name = models.CharField(max_length=20)
def __str__(self):
return self.name
If I use prefetch_related() to fetch the Seller objects along with their Genre and Book as following in one single databse query:
sellers = Seller.objects.prefetch_related('genre__book').filter()
However, I would like to filter out Seller objects that have no Book objects related to. What would be the syntax for the filter() in this case?
To filter genres that have no books in it you need the following condition:
genres = Genre.objects.exclude(pk__in=[x.genre.pk for x in Book.objects.all()]
To combine it with prefetch_related I think you need to use Prefetch object with a given queryset
from django.db.models import Prefetch
sellers = Seller.objects.prefetch_related(
Prefetch('genre_set',
queryset=Genre.objects.exclude(pk__in=[x.genre.pk for x in Book.objects.all()])
)
My question is associated with making querysets on models that are interconnected across many to many fields.
context - an app that a student enters and has to rate his or her teachers. the questions the app shows the student have the following logic: each student must rate some teachers. Each teacher has different categories of questions associated with them ("intelligence", "respect", "empathy",etc.) and each of these categories has some questions associated with it.
The models are:
class Items(models.Model):
item = models.TextField()
def __str__(self):
return self.item
class Categories(models.Model):
category = models.CharField(max_length=50,null=True)
items_associated = models.ManyToManyField(Items)
def __str__(self):
return self.category
class Professors(models.Model):
professor = models.CharField(max_length=50,null=True)
categories_assigned = models.ManyToManyField(Categories)
def __str__(self):
return self.professor
class Students(models.Model):
student_logged = models.CharField(max_length=50,null=True)
professors_to_evaluate = models.ManyToManyField(Professors)
def __str__(self):
return self.student_logged
when a student enters the web has some associated teachers (model Students) these teachers in turn have some categories assigned (model Professors), these categories in turn have some questions associated (model Categories). I want to store in a dictionary these questions that are in the model Items. How can I do it?
I've tried to filter and __in but I can't get it.
Many thanks and thank you for the wisdom
I highly suggest you use the related_name attribute. I've added _x to the related names to make the query more obvious.
class Items(models.Model):
item = models.TextField()
def __str__(self):
return self.item
class Categories(models.Model):
category = models.CharField(max_length=50,null=True)
items_associated = models.ManyToManyField(Items, related_name='category_x')
def __str__(self):
return self.category
class Professors(models.Model):
professor = models.CharField(max_length=50,null=True)
categories_assigned = models.ManyToManyField(Categories, related_name='professor_x')
def __str__(self):
return self.professor
class Students(models.Model):
student_logged = models.CharField(max_length=50,null=True)
professors_to_evaluate = models.ManyToManyField(Professors, related_name='student_x')
def __str__(self):
return self.student_logged
items_for_student = Items.objects.filter(category_x__professor_x__student_x=student)
But also the naming conventions you are using for the fields are a bit quirky. I've used best practices below so you can see what that would look like.
Don't have a field with the same name as the model
Models should be singular (with rare exceptions)
ManyToMany or ForeignKey relations should share the name of the model to make querying self document.
With those rules here is what the best practice looks like.
class Item(models.Model):
name = models.TextField()
def __str__(self):
return self.name
class Category(models.Model):
name = models.CharField(max_length=50,null=True)
items = models.ManyToManyField(Item, related_name='categories')
def __str__(self):
return self.name
class Professor(models.Model):
name = models.CharField(max_length=50,null=True)
categories = models.ManyToManyField(Category, related_name='professors')
def __str__(self):
return self.name
class Student(models.Model):
name = models.CharField(max_length=50,null=True)
professors = models.ManyToManyField(Professor, related_names='students')
def __str__(self):
return self.name
And with that structure the query would look like:
items = Item.objects.filter(categories__professors__students=student)
Also note that the above query will be very expensive to run on a database as it would evaluate to 3 joins.
I have the following models:
class Activity(models.Model):
name = models.CharField(max_length=128)
def __unicode__(self):
return self.name
class CustomerActivity(models.Model):
customer = models.ForeignKey(Customer)
activity = models.ForeignKey(Activity)
def __unicode__(self):
return self.activity.name
I have a filter that gets all the CustomerActivities for a customer:
customer_activities = CustomerActivity.objects.filter(customer=customer)
What I really need is all the Activity objects based on the results of the customer_activities?
Something like
activities = Activity.objects.filter(activity_in=customer_activities)???
Try this
activities = Activity.objects.filter(customeractivity__customer=customer)
Read more on lookups that span relationships
I'm trying to filter many-to-many relationship by some through Class field.
Quoting the Django documentation, i will explain my goal
class Person(models.Model):
name = models.CharField(max_length=128)
def __unicode__(self):
return self.name
class Group(models.Model):
name = models.CharField(max_length=128)
members = models.ManyToManyField(Person, through='Membership')
def __unicode__(self):
return self.name
class Membership(models.Model):
person = models.ForeignKey(Person)
group = models.ForeignKey(Group)
date_joined = models.DateField()
invite_reason = models.CharField(max_length=64)
In this example my goal sould be filter many to many relationship and obtain only the Person who has joined some Group starting from certain date (date_joined field).
Is it possible?
You can query across relationships with the django ORM (or in this case the reverse relationship):
person = Person.objects.filter(
membership__group=example_group,
membership__date_joined__gte=example_date
)
You can also do this:
person = example_group.members.filter(
membership__date_joined__gte=example_date
)
I have a model with two foreign keys to create many to many relationship - I am not using many to many field in Django
class Story(models.Model):
title = models.CharField(max_length=200)
pub_date = models.DateTimeField('date published')
def __unicode__(self):
return self.title
class Category(models.Model):
categoryText = models.CharField(max_length=50)
parentCat = models.ForeignKey('self',null=True,blank=True)
def __unicode__(self):
return self.categoryText
class StoryCat(models.Model):
story = models.ForeignKey(Poll,null=True,blank=True)
category = models.ForeignKey(Category,null=True,blank=True)
def __unicode__(self):
return self.story
I would like to query for a category like 'short', and retrieve all the unique keys to all stories returned.
>>>c=Category(categoryText='short')
>>>s=StoryCat(category=c)
when I try this I get errors "AttributeError: 'NoneType' object has no attribute 'title'. How can I do this?
I would like to query for a category like 'short', and retrieve all the unique keys to all stories returned.
c=Category.objects.get(categoryText='short')
story_ids = StoryCat.objects.filter(category=c).values_list('story')
And about your models:
Category name should probably be unique. And declare your many-to-many relation.
class Category(models.Model):
categoryText = models.CharField(max_length=50, unique=True)
stories = models.ManyToManyField(Story, through='StoryCat')
...
It makes no sense for intermediary table FK fields to be nullable. Also I assume the same story should not be added to the same category twice, so set a unique constraint.
class StoryCat(models.Model):
story = models.ForeignKey(Poll)
category = models.ForeignKey(Category)
class Meta:
unique_together = ('story', 'category')
The lines you're executing in the interpreter are not queries - they are instantiating new objects (but not saving them).
Presumably you meant this:
>>>c=Category.objects.get(categoryText='short')
>>>s=StoryCat.objects.get(category=c)