Reverse Look Up for Foreign Key using related_name in Django

Let’s understand the foreign key concept first.

Foreign Key in Django Model

Consider we have two models- Author and Books. The author field in the Books’ model is borrowed from the Author’ model.

This is how we can achieve it using the foreign key.

Model Author

class Author(models.Model):
    author_name = models.CharField(max_length=120)

Model Books

class Books(models.Model):
    book_name = models.CharField(max_length=120)
    writen_by = models.ForeignKey(Author, on_delete=models.CASCADE, null=True)

With the ForeignKey, we can set the one-to-many relationship between the two models.

If we have the instance of the Books model, we can access the Author information very easily. This is called forward compatibility.

Hope you are aware of reading model data.

Suppose book is the instance of the model Books. We can access the author name of the books in the Django template as below.

{{ book.author.author_name }}

If you want to access the foreign key value in the Django views, you can use the “__” notation.

For example,

Books.objects.all().values('book_name', 'writen_by__author_name')

But what if you want to access the Books information from the Author instance? There is a special provision in Django to accomplish this so-called backward compatibility.

Reverse Look Up for Foreign Key (aka Backward Compatibility)

For example, we want to list down the name of all the books written by the specific author.

The simple way of doing is that, update ForeignKey field in the Books model by setting related_name parameter. Give it any suitable name (say authors)

class Books(models.Model):
    book_name = models.CharField(max_length=120)
    author = models.ForeignKey(Author, on_delete=models.CASCADE, null=True, related_name="authors")

Note: If you are modifying the model, don’t forget to make the migration.

Now, with the instance of the Author model, we can access information in the Django template about the books s/he has written using related_name.

Let’s say the author is the instance of the Author model.

How to get the list of all the books written by the author?

 {{ author.authors.book_name.all }} 

One author can have multiple books written. So it will return the list of the books.

You can print all the book names written by the author.

{% for book in author.authors.book_name.all }}
    {{book.book_name}}
{% endfor %} 

Find the name of the First Book written by the author.

Use keyword first.

 {{ author.authors.book_name.first }}  

Find the name of the Last Book written by the author.

Use keyword last.

 {{ author.authors.book_name.last }}  

With this simple reverse look-up using related_name, accessing foreign key information has become very easy. Isn’t it?

Note: If you don’t set related_name, Django has default related_name i.e books_set.

This is all about Reverse Look Up for Foreign Key using related_name in Django. I hope, with this tutorial you have learnt about backward compatibility. If you have any doubts or questions to ask, write in the comment. Thank you!