Master Django ORM: Advanced Foreign Key Queries & Reverse Lookups

Review

Previously we covered the basic filter() query operation in Django ORM.

Query Operations

Object.foreign_key_field

When we have a Book instance, we can print its related fields directly.

book = models.Book.objects.filter(title="<<大明帝国>>").first()
print(f"book type: {type(book)}")
print(f"id: {book.id}")
print(f"title: {book.title}")
print(f"price: {book.price}")
print(f"publish date: {book.PublishDate}")
print(f"publisher: {book.publish}")  # foreign‑key field

Result

Note: the blue text indicates a foreign‑key field.

The reason book.publish prints the publisher name is that the Publish model defines __str__ to return self.title. To access other publisher attributes, we can use the foreign‑key object directly.

print(f"Publisher type: {type(book.publish)}")  # <class 'web.models.Publish'>
print(f"Publisher phone: {book.publish.phone},")

Reverse Query (model__set.all())

We can retrieve all books belonging to a specific publisher by first obtaining the publisher object and then using the reverse relation.

# Query publisher
publish_obj = models.Publish.objects.filter(title="邮电出版社").first()
# Get publisher id
publish_id = publish_obj.id
# Find books with that publisher id
book_list = models.Book.objects.filter(publish_id=publish_id)
print(book_list)

Alternatively, Django provides a shortcut using the reverse manager:

publish_obj = models.Publish.objects.filter(title="邮电出版社").first()
book_list = publish_obj.book_set.all()
print(book_list)

Double‑Underscore Cross‑Table Query

Using the double‑underscore syntax we can filter books by a publisher's title in a single query.

book_list = models.Book.objects.filter(publish__title="邮电出版社")
print(book_list)

The double‑underscore can also be chained for multi‑table lookups, e.g., retrieving books and their many‑to‑many authors through the publisher:

ret = models.BookManyAuthor.objects.filter(book__publish__title="邮电出版社")
print(ret)

values

When only specific columns are needed, values() returns dictionaries of the selected fields.

# Syntax
book_list = models.Book.objects.all().values("title", "price")
print(book_list)

values_list

values_list()

works like values() but returns tuples instead of dictionaries.

book_list = models.Book.objects.all().values_list("title", "price")
print(book_list)

related_name

The related_name attribute customizes the reverse manager name, replacing the default model_set.

Original way (using book_set):

publish = models.Publish.objects.filter(title="邮电出版社").first()
book_list = publish.book_set.all()
print(book_list)

Alias way (using book_list as defined by related_name='book_list'):

publish = models.Publish.objects.filter(title="邮电出版社").first()
book_list = publish.book_list.all()
print(book_list)

Chaining Multiple filter() Calls

Multiple filter() calls can be chained to apply successive conditions, useful when filter criteria are built dynamically.

book1 = models.Book.objects.filter(title="<<大明帝国>>").filter(price="99")
book2 = models.Book.objects.filter(title="<<大明帝国>>", price="99")
print(book1)
print(book2)

Summary

This article extends the previous discussion on filter queries, focusing on foreign‑key field access, reverse lookups using the default model__set and custom related_name, double‑underscore cross‑table filters, chained filters, and the differences between values and values_list.