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Discover effective strategies to handle `prefetch_related` queries in Django when it comes to model properties like business phone numbers. Enhance your Django Admin performance with these tips!
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This video is based on the question https://stackoverflow.com/q/73734880/ asked by the user 'TamaraB' ( https://stackoverflow.com/u/5407623/ ) and on the answer https://stackoverflow.com/a/73735008/ provided by the user 'Maxim Danilov' ( https://stackoverflow.com/u/18728010/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Solving Django Admin Prefetch Related Queries for Properties in Models

When working with Django models, especially in the context of the Django admin interface, you may come across situations that can be puzzling, particularly when prefetching related objects. In this post, we address a specific problem: how to efficiently retrieve a property from a model using the prefetch_related method without causing multiple queries to the database.

The Problem Overview

Imagine you have two Django models: Merchant and PhoneNumber. The PhoneNumber model includes a foreign key linking back to the Merchant model, and the Merchant model has a property called business_phone_number which retrieves the first phone number associated with it. When you add business_phone_number to the Django admin’s list_display, you might run into a performance issue where the database queries multiply, sometimes running into the hundreds.

Let’s review the relevant parts of the code:

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

In this instance, it’s common to use the prefetch_related method in the get_queryset to fetch related objects efficiently:

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

Unfortunately, despite this setup, the problem persists where you find that Django is still querying the PhoneNumber table multiple times, leading to inefficiencies.

Understanding the Issue

The root of the problem lies in how you access the first phone number within your property. When you call self.phonenumber_set.first(), Django does not utilize the prefetched data. Instead, it executes an individual query each time it's invoked.

Why This Happens

self.phonenumber_set.first() triggers a new query to the database because it does not leverage the already prefetched set of results.

The correct way to work with a prefetched queryset is to access the data that has already been fetched.

The Solution

To fix this, you should change how you access the prefetched data in your business_phone_number property. Here’s how:

Option 1: Iterate through the prefetched queryset

Rather than using first(), you can utilize all() to iterate over the prefetched results:

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

Option 2: Use a Generator Expression

Alternatively, you can use Python’s next() function with a generator expression to achieve the same result more succinctly:

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

Option 3: Using iterator()

Another approach, though less common in typical usage, is using iterator(), which is efficient for large query sets:

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

This ensures that your code works effectively with the data already loaded into memory rather than fetching it from the database repeatedly.

Conclusion

By understanding how Django handles queries on properties tied to related models and modifying how those properties access their underlying data, you can drastically improve the performance of your Django admin interface. Avoid pitfalls that lead to repeated database queries by leveraging prefetched results effectively.

By implementing these best practices, you'll not only optimize your Django application but also enhance the user experience on the admin side. Happy coding!