models.py:

from django.db import models


# Press
class Publisher(models.Model):
    id = models.AutoField(primary_key=True)
    name = models.CharField(max_length=64, null=False, unique=True)

    def __str__(self):
        return "<Publisher object: {}>".format(self.name)


# book
class Book(models.Model):
    id = models.AutoField(primary_key=True)
    title = models.CharField(max_length=64, null=False, unique=True)
    publisher = models.ForeignKey(to="Publisher")

    def __str__(self):
        return "<Book object: {}>".format(self.title)


# author
class Author(models.Model):
    id = models.AutoField(primary_key=True)
    name = models.CharField(max_length=16, null=False, unique=True)
    book = models.ManyToManyField(to="Book")  # The ORM automatically generates the third table with many-to-many associative Book tables

    def __str__(self):
        return "<Author object: {}>".format(self.name)

book table:

author table:

The third table associated with author and book, author_book table:

Pusher table:

 

Many-to-many queries:

orm.py:

import os

if __name__ == '__main__':
    # Loading configuration information for Django projects
    os.environ.setdefault("DJANGO_SETTINGS_MODULE", "mysite2.settings")
    # Import Django and start the Django project
    import django
    django.setup()

    from app01 import models

    author_obj = models.Author.objects.get(id=2)  # Author who gets id 2
    ret = author_obj.book.all()  # Query all books written by authors with id 2
    print(ret)

Operation results:

 

create():

Create a new object, save the object, add it to the associated object set, and return the newly created object.

orm.py:

import os

if __name__ == '__main__':
    # Loading configuration information for Django projects
    os.environ.setdefault("DJANGO_SETTINGS_MODULE", "mysite2.settings")
    # Import Django and start the Django project
    import django
    django.setup()

    from app01 import models

    author_obj = models.Author.objects.get(id=1)  # Get the author with id 1
    author_obj.book.create(title="<PHP>", publisher_id=2)  # Create a book through the author

Operation results:

 

add():

Adds the specified model object to the associated object set

orm.py:

import os

if __name__ == '__main__':
    # Loading configuration information for Django projects
    os.environ.setdefault("DJANGO_SETTINGS_MODULE", "mysite2.settings")
    # Import Django and start the Django project
    import django
    django.setup()

    from app01 import models

    # Add a book with id 2 to the author with id 4
    author_obj = models.Author.objects.get(id=4)
    book_obj = models.Book.objects.get(id=2)
    author_obj.book.add(book_obj)
    
    # You can also add it directly with id
    # author_obj.book.add(2)

Operation results:

Add more than one

orm.py:

import os

if __name__ == '__main__':
    # Loading configuration information for Django projects
    os.environ.setdefault("DJANGO_SETTINGS_MODULE", "mysite2.settings")
    # Import Django and start the Django project
    import django
    django.setup()

    from app01 import models

    # Add more than one book with id greater than 4 to the author with id 3
    author_obj = models.Author.objects.get(id=3)
    book_objs = models.Book.objects.filter(id__gt=4)
    author_obj.book.add(*book_objs)   # Use * to break up the list and pass it in

Operation results:

 

remove():

Remove the executed model object from the set of associated objects.

orm.py:

import os

if __name__ == '__main__':
    # Loading configuration information for Django projects
    os.environ.setdefault("DJANGO_SETTINGS_MODULE", "mysite2.settings")
    # Import Django and start the Django project
    import django
    django.setup()

    from app01 import models

    author_obj = models.Author.objects.get(id=3)
    book_obj = models.Book.objects.get(title="<PHP>")
    author_obj.book.remove(book_obj)  # Delete the association with title field PHP
    author_obj.book.remove(5)  # Direct deletion

Operation results:

Two books associated with the author of the original id 3 have been deleted

 

clear():

Clear all objects from the set of associated objects.

orm.py:

import os

if __name__ == '__main__':
    # Loading configuration information for Django projects
    os.environ.setdefault("DJANGO_SETTINGS_MODULE", "mysite2.settings")
    # Import Django and start the Django project
    import django
    django.setup()

    from app01 import models

    # Empty all the books of authors with id 4
    author_obj = models.Author.objects.get(id=4)
    author_obj.book.clear()

Operation results:

The content of author_id=4 is completely emptied

 

set():

Update the associated object of the model object.

orm.py:

import os

if __name__ == '__main__':
    # Loading configuration information for Django projects
    os.environ.setdefault("DJANGO_SETTINGS_MODULE", "mysite2.settings")
    # Import Django and start the Django project
    import django
    django.setup()

    from app01 import models

    # Add two books with id 2 and 3 to authors with id 4
    author_obj = models.Author.objects.get(id=4)
    author_obj.book.set([2, 3])

Operation results:

 

Foreign key supplement:

clear() and remove() methods in foreign keys can only be used when null=True

Book classes in models.py:

# book
class Book(models.Model):
    id = models.AutoField(primary_key=True)
    title = models.CharField(max_length=64, null=False, unique=True)
    publisher = models.ForeignKey(to="Publisher", null=True)  # Set null to True

    def __str__(self):
        return "<Book object: {}>".format(self.title)

orm.py:

import os

if __name__ == '__main__':
    # Loading configuration information for Django projects
    os.environ.setdefault("DJANGO_SETTINGS_MODULE", "mysite2.settings")
    # Import Django and start the Django project
    import django
    django.setup()

    from app01 import models

    # Empty the books in the publishing house with id 2
    publisher_obj = models.Publisher.objects.get(id=2)
    publisher_obj.book_set.clear()

Operation results:

Pusher_id is changed from 2 to 0