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Jun 25, 2025
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Add FAQ about slow joins #320

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13 changes: 13 additions & 0 deletions docs/source/faq.rst
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Expand Up @@ -4,6 +4,19 @@ FAQ

This page contains a list of some frequently asked questions.

Performance
===========

Querying across relational fields like :class:`~django.db.models.ForeignKey` and :class:`~django.db.models.ManyToManyField` is really slow. Is there a way to improve the speed of these joins?
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Not really. Joins use MongoDB's :doc:`$lookup
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Re: "Not really."

We could mention the possibility of using raw_aggregate() to manually construct a query as mentioned in #309, but I think that's unlikely to be an attractive and maintainable solution for most projects.
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🤔 we could, the people then will ask: why aren't you doing that approach?
The answer is: because we have to modify the django query tree or handle this particular case differently. IMO keep this as it is now, otherwise we should explain the limitations that we have in this particular case.
<manual:reference/operator/aggregation/lookup>` operator, which doesn't perform
well with large tables.

The best practice for modeling relational data in MongoDB is to instead use
:doc:`embedded models <topics/embedded-models>`.

Troubleshooting
===============

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