model - Django Queryset with filtering on reverse foreign key

Model - Django Queryset with filtering on reverse foreign key

In Django, querying based on reverse foreign key relationships involves using the double underscore (__) notation to navigate across relationships. This notation allows you to filter and access related objects efficiently. Let's go through an example to illustrate how to use queryset filtering with reverse foreign keys.

Example Models Setup

Consider a scenario where you have two models Author and Book, where each Book is associated with an Author through a foreign key relationship:

from django.db import models class Author(models.Model): name = models.CharField(max_length=100) class Book(models.Model): title = models.CharField(max_length=200) author = models.ForeignKey(Author, on_delete=models.CASCADE, related_name='books') published_year = models.IntegerField()

Querying with Reverse Foreign Key

To filter Book objects based on properties of the related Author, you can use the related_name defined in the foreign key field (books in this case) to access the reverse relationship from Author to Book.

Example Queries:

  1. Filter books by author name:

    from django.shortcuts import get_object_or_404 from myapp.models import Author, Book # Assuming you have an author_id from somewhere author_id = 1 # Get the author object author = get_object_or_404(Author, pk=author_id) # Filter books by author's name books = author.books.filter(title__icontains='python')

    This query filters all books written by authors whose name contains "python".

  2. Filter books by published year of the author:

    from django.shortcuts import get_object_or_404 from myapp.models import Author, Book # Assuming you have an author_id from somewhere author_id = 1 # Get the author object author = get_object_or_404(Author, pk=author_id) # Filter books by published year of the author books = author.books.filter(published_year__gte=2000)

    This query filters all books written by authors who published a book after the year 2000.

Explanation:

  • related_name Attribute: The related_name='books' in the ForeignKey field (author) allows you to access the reverse relationship from Author to Book. So, author.books gives you a queryset of all books associated with that author.

  • Double Underscore (__) Syntax: Allows you to navigate across relationships in Django's ORM. For example, author.books.filter(...) navigates from Author to Book and applies filters based on Book attributes.

Conclusion:

Using Django's queryset filtering with reverse foreign keys (related_name) allows you to efficiently query and filter objects based on properties of related models. Ensure that your models are properly defined with appropriate foreign key relationships and related_name attributes to leverage Django's powerful ORM for querying related data. Adjust the filters (title__icontains, published_year__gte, etc.) according to your specific application's requirements and data model.

Examples

  1. Django queryset filter by reverse foreign key

    Description: Querying a Django model based on a condition on its reverse foreign key relationship.

    # Example: Filtering queryset by reverse foreign key from django.db import models class Author(models.Model): name = models.CharField(max_length=100) class Book(models.Model): title = models.CharField(max_length=100) author = models.ForeignKey(Author, on_delete=models.CASCADE, related_name='books') # Query to get authors who have books published after 2020 authors = Author.objects.filter(books__publish_date__gt='2020-01-01')

  2. Django queryset exclude reverse foreign key

    Description: Excluding objects in a Django queryset based on a condition on their reverse foreign key relationship.

    # Example: Excluding queryset by reverse foreign key from django.db import models class Author(models.Model): name = models.CharField(max_length=100) class Book(models.Model): title = models.CharField(max_length=100) author = models.ForeignKey(Author, on_delete=models.CASCADE, related_name='books') # Query to exclude authors who have books with 'fiction' genre authors = Author.objects.exclude(books__genre='fiction')

  3. Django queryset annotate reverse foreign key count

    Description: Annotating a Django queryset with the count of related objects through a reverse foreign key.

    # Example: Annotating queryset with reverse foreign key count from django.db import models from django.db.models import Count class Author(models.Model): name = models.CharField(max_length=100) class Book(models.Model): title = models.CharField(max_length=100) author = models.ForeignKey(Author, on_delete=models.CASCADE, related_name='books') # Query to annotate authors with the count of their books authors = Author.objects.annotate(num_books=Count('books'))

  4. Django queryset filter by reverse foreign key attribute

    Description: Filtering a Django queryset based on an attribute of its reverse foreign key related objects.

    # Example: Filtering queryset by reverse foreign key attribute from django.db import models class Author(models.Model): name = models.CharField(max_length=100) class Book(models.Model): title = models.CharField(max_length=100) author = models.ForeignKey(Author, on_delete=models.CASCADE, related_name='books') publish_date = models.DateField() # Query to get authors who have books published after 2020 authors = Author.objects.filter(books__publish_date__gt='2020-01-01')

  5. Django queryset filter by reverse foreign key count

    Description: Filtering a Django queryset based on the count of related objects through a reverse foreign key.

    # Example: Filtering queryset by reverse foreign key count from django.db import models class Author(models.Model): name = models.CharField(max_length=100) class Book(models.Model): title = models.CharField(max_length=100) author = models.ForeignKey(Author, on_delete=models.CASCADE, related_name='books') # Query to get authors who have more than 3 books authors = Author.objects.annotate(num_books=models.Count('books')).filter(num_books__gt=3)

  6. Django queryset filter by reverse foreign key exists

    Description: Filtering a Django queryset based on the existence of related objects through a reverse foreign key.

    # Example: Filtering queryset by reverse foreign key exists from django.db import models class Author(models.Model): name = models.CharField(max_length=100) class Book(models.Model): title = models.CharField(max_length=100) author = models.ForeignKey(Author, on_delete=models.CASCADE, related_name='books') # Query to get authors who have at least one book authors = Author.objects.filter(books__isnull=False).distinct()

  7. Django queryset filter by reverse foreign key aggregate

    Description: Filtering a Django queryset based on an aggregate function of related objects through a reverse foreign key.

    # Example: Filtering queryset by reverse foreign key aggregate from django.db import models class Author(models.Model): name = models.CharField(max_length=100) class Book(models.Model): title = models.CharField(max_length=100) author = models.ForeignKey(Author, on_delete=models.CASCADE, related_name='books') rating = models.DecimalField(max_digits=3, decimal_places=2) # Query to get authors whose books' average rating is above 4.0 authors = Author.objects.annotate(avg_rating=models.Avg('books__rating')).filter(avg_rating__gt=4.0)

  8. Django queryset filter by reverse foreign key max/min value

    Description: Filtering a Django queryset based on the maximum or minimum value of a field in related objects through a reverse foreign key.

    # Example: Filtering queryset by reverse foreign key max/min value from django.db import models class Author(models.Model): name = models.CharField(max_length=100) class Book(models.Model): title = models.CharField(max_length=100) author = models.ForeignKey(Author, on_delete=models.CASCADE, related_name='books') publish_year = models.IntegerField() # Query to get authors whose latest book was published in 2022 authors = Author.objects.filter(books__publish_year=models.Max('books__publish_year'))

  9. Django queryset filter by reverse foreign key range

    Description: Filtering a Django queryset based on a range of values from related objects through a reverse foreign key.

    # Example: Filtering queryset by reverse foreign key range from django.db import models class Author(models.Model): name = models.CharField(max_length=100) class Book(models.Model): title = models.CharField(max_length=100) author = models.ForeignKey(Author, on_delete=models.CASCADE, related_name='books') pages = models.IntegerField() # Query to get authors whose books have more than 200 pages authors = Author.objects.filter(books__pages__gt=200)

  10. Django queryset filter by reverse foreign key multiple conditions

    Description: Filtering a Django queryset based on multiple conditions on related objects through a reverse foreign key.

    # Example: Filtering queryset by reverse foreign key multiple conditions from django.db import models class Author(models.Model): name = models.CharField(max_length=100) class Book(models.Model): title = models.CharField(max_length=100) author = models.ForeignKey(Author, on_delete=models.CASCADE, related_name='books') publish_date = models.DateField() genre = models.CharField(max_length=50) # Query to get authors who have science fiction books published after 2020 authors = Author.objects.filter(books__genre='science fiction', books__publish_date__gt='2020-01-01')

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