Understanding Django Migration Errors with FilterSet

Based on your description, you encountered an issue where Django was trying to access database tables during the migration process, causing "no such table" errors. This is a common problem when code that runs during Django's initialization attempts to query the database before it's fully set up.

Why This Happens

When Django starts up, it loads all your application code, including models, views, and other components. During this process, any code that gets executed immediately (not just defined) might try to access the database. This includes:

1. Class definitions with attributes that execute queries
2. Module-level code that runs database queries
3. Import-time evaluations that depend on database access

In your case, the FilterSet class was likely being initialized during Django's startup process, and it was trying to execute a list comprehension that queried the database to populate filter choices.

The Problem with List Comprehensions in FilterSets

When you use a list comprehension directly in a FilterSet class definition like:

PYTHONCopied
class MyFilterSet(FilterSet):
    my_field = ChoiceFilter(
        choices=[(obj.id, obj.name) for obj in SomeModel.objects.all()]
    )

This code executes immediately when the module is imported, not when the filter is actually used. During migrations or test setup, the tables might not exist yet, causing the error.

The Solution

The proper approach is to use lazy evaluation for these choices, either by:

1. Using ModelChoiceFilter which handles the queryset properly
2. Using a callable for the choices parameter
3. Overriding the __init__ method to set the choices at runtime

This defers database access until the filter is actually used, not when the class is defined.

Why Your Fix Worked

Without seeing your exact code change, I can infer that you likely changed from an eager evaluation (list comprehension that runs at import time) to a lazy evaluation approach that only queries the database when the filter is actually used.

This is a fundamental principle in Django: avoid database queries during module import or class definition. Instead, use methods, callables, or specialized filter types that perform database operations only when needed.

For migrations specifically, Django provides the --skip-checks option precisely to bypass code that might try to access the database during the migration process, which is why that workaround helped temporarily.