Implementing Complex JOIN Conditions in Django ORM

To implement the SQL query with the IS NOT DISTINCT FROM condition in Django ORM, you'll need to use a custom lookup or a more complex query construction. The IS NOT DISTINCT FROM operator is particularly useful because it treats NULL values as equal when they appear in both sides of the comparison.

Solution Approach

You can implement this using Django's Q objects and custom conditions. Here's how you might approach it:

PYTHONCopied
from django.db.models import Q, OuterRef, Subquery, F

# First, create a subquery that includes the condition
sweet_subquery = Sweet.objects.filter(
    fruit_id=OuterRef('fruit_id'),
    # This handles the IS NOT DISTINCT FROM logic
    # Both NULL or both equal values
).filter(
    Q(category_id=F('food__category_id')) | 
    (Q(category_id__isnull=True) & Q(food__category_id__isnull=True))
)

# Then use this in your main query
result = Food.objects.annotate(
    sweet_id=Subquery(sweet_subquery.values('id')[:1])
)

Alternative Approach with Raw SQL

If the above approach doesn't work for your specific case, you might need to use raw SQL:

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from django.db import models

Food.objects.raw("""
    SELECT food.*
    FROM food
    LEFT JOIN sweet
        ON (sweet.fruit_id = food.fruit_id) AND
           (sweet.category_id IS NOT DISTINCT FROM food.category_id)
""")

Custom Lookup Implementation

For a more reusable solution, you could create a custom lookup for the IS NOT DISTINCT FROM operator:

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from django.db.models import Lookup

class NotDistinctFrom(Lookup):
    lookup_name = 'not_distinct_from'

    def as_sql(self, compiler, connection):
        lhs, lhs_params = self.process_lhs(compiler, connection)
        rhs, rhs_params = self.process_rhs(compiler, connection)
        params = lhs_params + rhs_params
        return "%s IS NOT DISTINCT FROM %s" % (lhs, rhs), params

# Register the lookup
from django.db.models.fields import Field
Field.register_lookup(NotDistinctFrom)

# Then use it in your query
Food.objects.annotate(
    sweet_id=Subquery(
        Sweet.objects.filter(
            fruit_id=OuterRef('fruit_id'),
            category_id__not_distinct_from=F('food__category_id')
        ).values('id')[:1]
    )
)

Note that the IS NOT DISTINCT FROM operator might not be supported in all database backends, so you may need to adjust the SQL generation based on your specific database.