Many-to-Many

A Many-to-Many relationship occurs when each record in a model can be associated with many records in another model, and vice versa. This is facilitated by Django's models.ManyToManyField.

Let's explore the ManyToManyField with an example. In our project, we use two models: Author and Book. An author can write multiple books, and a book might be co-authored by multiple authors. This is a classic many-to-many relationship.

Let's implement it in our code. In the models.py add to the Book model a new field.

Accessing Related Records

From Author to Books:

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author = Author.objects.get(pen_name="Richard Bachman")
books_written = author.book_set.all()  # get all books written by this author

From Book to Authors:

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book = Book.objects.get(title="Black House")
authors_of_book = book.authors.all()  # get all authors of this book

Adding and Removing Relationships

Adding an author to a book:

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author = Author.objects.get(pen_name="Richard Bachman")
book = Book.objects.
book.authors.add(author)

Removing an author from a book:

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book.authors.remove(author)

Filtering with ManyToManyField

You can also filter queries based on Many-to-Many relationships. For example, to find all books co-authored by a specific author:

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books = Book.objects.filter(authors__pen_name="Richard Bachman")