A Django ORM Subquery Expression which only returns the specified fields

django_subquery_only.py

from django.db.models import Subquery


class SubquerySelect(Subquery):
    """Return only a subset of fields from a Subquery"""

    template = '(SELECT %(fields)s FROM (%(subquery)s) AS %(subquery_alias)s)'

    def __init__(self, queryset, output_field=None, fields=('*',), **extra):
        self.fields = tuple(fields)
        extra['fields'] = ', '.join(fields)
        extra.setdefault('subquery_alias', 'subquery')
        super().__init__(queryset, output_field=output_field, **extra)

    def copy(self):
        clone = super().copy()
        clone.fields = self.fields
        return clone

    def get_source_expressions(self):
        return super().get_source_expressions() + list(self.fields)