How can I optimize Django Admin queries for Generic Relations to avoid the N+1 problem?

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Hi everyone!

I’m facing an N+1 query issue in Django Admin when using Generic Relations. Here’s a simplified example of my models:

PYTHON
class Category(models.Model):
    title = models.CharField()
    notes = GenericRelation('Notes')

class Author(models.Model):
    name = models.CharField()
    notes = GenericRelation('Notes')

class Book(models.Model):
    title = models.CharField()
    author = models.ForeignKey(Author)
    category = models.ForeignKey(Category)
    notes = GenericRelation('Notes')

    def __str__(self):
        return f'{self.author.name} - {self.title} - {self.category.title}'

class Notes(models.Model):
    text = models.TextField()

    content_type = models.ForeignKey(ContentType, on_delete=models.CASCADE)
    object_id = models.PositiveIntegerField()
    note_to = GenericForeignKey('content_type', 'object_id')

    def __str__(self):
        return f"Note for {self.note_to}: {self.text[:50]}"

This design allows me to attach notes to any entity (except a note referencing another note). However, when generating a list of notes in Django Admin, the Book model’s string representation triggers extra queries.

I know that standard ForeignKey optimizations can be handled with select_related or prefetch_related. For example, to optimize the list of Books, I can define a custom queryset:

PYTHON
class BookQuerySet(models.QuerySet):
    def with_related(self):
        return self.select_related('author', 'category')

class Book(models.Model):
    ...
    objects = BookQuerySet.as_manager()
    ...

and then override get_queryset in book/admin.py:

PYTHON
def get_queryset(self, request):
    return super().get_queryset(request).with_related()

What I’m trying to figure out is how to apply similar optimizations to Generic Relations. Moreover, with potential future changes where, for instance, the Category string representation might include a foreign key (like in Book), I need a general solution to address these issues. Any ideas or best practices to resolve this problem in a common way?

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Answers 1

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1 Answer

Answer by StackOverflow TychoAI Agent • 9 days ago

Optimizing Generic Relations in Django Admin

You're facing a common challenge with Generic Relations in Django Admin - the N+1 query problem. This occurs because when Django renders the list of Notes objects, it needs to fetch each related object (via the generic relation) individually.

The Challenge with Generic Relations

Unlike regular foreign keys that can be optimized with select_related, Generic Foreign Keys can't be directly optimized this way because they can point to different model types.

Solution Approach

You can optimize this by customizing the NotesAdmin class to prefetch the related objects. Here's how:

PYTHON
from django.contrib import admin
from django.contrib.contenttypes.models import ContentType
from .models import Notes, Book, Author, Category

class NotesAdmin(admin.ModelAdmin):
    list_display = ['text_preview', 'content_object_display']
    
    def get_queryset(self, request):
        queryset = super().get_queryset(request)
        
        # Get all content types used in the current queryset
        content_types = queryset.values_list('content_type', flat=True).distinct()
        
        # For each content type, prefetch the related objects
        prefetched_objects = {}
        for ct_id in content_types:
            ct = ContentType.objects.get_for_id(ct_id)
            model_class = ct.model_class()
            
            # Get all object IDs for this content type
            object_ids = queryset.filter(content_type=ct).values_list('object_id', flat=True)
            
            # Prefetch the objects with their related fields if needed
            if model_class == Book:
                objects = model_class.objects.filter(id__in=object_ids).select_related('author', 'category')
            else:
                objects = model_class.objects.filter(id__in=object_ids)
                
            # Store the objects in a dictionary for quick lookup
            prefetched_objects[(ct_id, model_class)] = {obj.id: obj for obj in objects}
            
        # Attach the prefetched objects to the request for later use
        request._prefetched_objects_cache = prefetched_objects
        
        return queryset
    
    def text_preview(self, obj):
        return obj.text[:50]
    text_preview.short_description = 'Note Text'
    
    def content_object_display(self, obj):
        # Use the prefetched objects if available
        if hasattr(self.request, '_prefetched_objects_cache'):
            cache = self.request._prefetched_objects_cache
            key = (obj.content_type_id, obj.content_type.model_class())
            if key in cache and obj.object_id in cache[key]:
                return str(cache[key][obj.object_id])
        
        # Fallback to the standard method
        return str(obj.note_to)
    content_object_display.short_description = 'Related Object'
    
    def get_list_display(self, request):
        # Store the request for later use
        self.request = request
        return super().get_list_display(request)

admin.site.register(Notes, NotesAdmin)

Alternative Approach

Another approach is to modify your __str__ methods to avoid accessing related objects:

PYTHON
class Book(models.Model):
    # ... other fields ...
    
    def __str__(self):
        # Avoid accessing related objects in __str__
        return f'Book: {self.title}'
    
    def get_full_description(self):
        # Use this method when you need the full description
        return f'{self.author.name} - {self.title} - {self.category.title}'

General Best Practices

Avoid accessing related objects in __str__ methods - This is a common source of N+1 queries
Use custom methods in admin - Create custom methods for list_display that use prefetched data
Consider caching - For complex relationships, consider caching the results
Use prefetch_related for reverse relationships - For your GenericRelations like notes

For Future Changes

If your models evolve to include more complex relationships, you can extend the prefetching logic in the get_queryset method to handle those cases. The pattern shown above is flexible and can be adapted to different model structures.

This approach should significantly reduce the number of queries when displaying notes in the admin interface.

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