How Can I Efficiently Find and Retrieve Rows with Duplicate Field Values in Django Models?

Querying Django Models for Duplicate Field Values

In Django, dealing with models that allow duplicate field values can pose challenges. Consider a model with a non-unique name field. To select rows with duplicate name values, here are possible solutions:

Plain SQL Solution

The provided SQL query effectively filters for rows with duplicate names:

select * from literal where name IN (
    select name from literal group by name having count((name)) > 1
);

Django ORM Solution

While Django ORM doesn't provide a direct method for duplicate field value filtering, you can achieve it with some ingenuity:

from django.db.models import Count

dupes = Literal.objects.values('name')\
                       .annotate(Count('id'))\
                       .order_by()\
                       .filter(id__count__gt=1)

This code creates a query that groups rows by their name field, counts the occurrences of each unique name, and filters for rows with more than one occurrence.

Enhancing the Django ORM Solution

To retrieve actual model objects instead of a values query set, you can use the in operator:

Literal.objects.filter(name__in=[item['name'] for item in dupes])

This code constructs a new query that filters the Literal model by names found in the dupes query set.

This approach provides a more Django-like solution while maintaining the accuracy of the plain SQL query. It's worth noting that this method may have performance implications if the number of duplicate rows is significant. In such cases, using raw SQL with database-specific optimizations might be more efficient.

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