Скачать или смотреть Efficiently Fetching Querysets of Related Objects in Django

Learn how to retrieve a `queryset` of all persons in a company using Django's ORM effectively. Avoid slow loops and get results efficiently with a single query!
---
This video is based on the question https://stackoverflow.com/q/64141184/ asked by the user 'ProfHase85' ( https://stackoverflow.com/u/1945486/ ) and on the answer https://stackoverflow.com/a/64141353/ provided by the user 'Lemayzeur' ( https://stackoverflow.com/u/5644965/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Django queryset of related objects

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Efficiently Fetching Querysets of Related Objects in Django

When working with Django, fetching related objects efficiently is essential for performance. If you're developing a system involving companies, departments, and persons, you might encounter a common problem: retrieving a list of persons associated with a specific company.

Let's unravel how to achieve this in a more efficient manner using Django's powerful ORM capabilities.

Understanding the Models

Before diving into querying, let's briefly examine the model structure:

[[See Video to Reveal this Text or Code Snippet]]

In this setup:

Company has a name.

Department links to Company via a foreign key and has a many-to-many relationship with Person.

Person also has a name.

With this in mind, you may want to get all persons in a specific company. A straightforward (but inefficient) approach would be to loop through departments of the company and gather persons.

The Inefficient Approach

Here's an example of a function you might have considered, but there are significant downsides:

[[See Video to Reveal this Text or Code Snippet]]

Why Is This Inefficient?

Performance: Looping through multiple departments and persons can lead to a slow execution time, especially with a large dataset.

List Instead of Queryset: This method returns a list, which is not filterable or chainable like a queryset.

The Efficient Solution

Instead of using nested loops, you can utilize Django's ORM to create a queryset directly. Here's how:

Fetching Persons Using a Queryset

The efficient solution takes advantage of Django's filtering and distinct methods. You can retrieve persons linked to a company in a single line:

[[See Video to Reveal this Text or Code Snippet]]

Breaking it Down:

Person.objects.filter(...): This initiates a lookup on the Person model.

department__company__name__iexact=company_name: This traverses the relationships, filtering persons by the company name (case-insensitive).

.distinct(): This ensures that you don't retrieve duplicate entries in the case that multiple departments might have the same persons.

Benefits of the Efficient Approach

Speed: The query is executed at the database level, making it much faster than the initial looping method.

Queryset: The returned object is a queryset, allowing further filtering, ordering, or chaining additional queries.

Simplicity: The code is concise and clear, making it maintainable and easier to read.

Conclusion

By leveraging Django's ORM correctly, you can significantly improve the performance of your data retrieval processes. The one-liner provided is not only cleaner but also enhances maintainability and scalability of your application. Next time you need to pull related objects, consider using a queryset for optimal efficiency!

Feel free to test out the examples and see how Django can simplify your database interactions while providing robust performance.