mytables = Table.objects.filter(order_table__date=date)

from django.db.models import Count
from django.db.models import Q
Table.objects.annotate(cnt=Count('order_table', filter=Q(order_table__date=date))).filter(cnt__gt=0)

def check_free_tables(request):
    from django.db.models import Count
    from django.db.models import Q
    
    if request.is_ajax and request.method == 'GET':
        date = request.GET.get('date')
        mytables = Table.objects.annotate(cnt=Count('order_table', filter=Q(order_table__date=date))).filter(cnt__gt=0)
        print(mytables)
        ser_instance = serializers.serialize('json', list(mytables), fields=('id', 'is_reserved'))
        print(ser_instance)
        return JsonResponse({'tables': ser_instance}, status=200, safe=False)

print(Table.objects.annotate(cnt=Count('order_table', filter=Q(order_table__date=date))).filter(cnt__gt=0).query)

SELECT "restaurant_table"."id", "restaurant_table"."hall_id", "restaurant_table"."shape", "restaurant_table"."number", "restaurant_table"."seats", "restaurant_table"."width", "restaurant_table"."height", "restaurant_table"."coordinate_x", "restaurant_table"."coordinate_y", "restaurant_table"."is_reserved", C
OUNT(CASE WHEN "restaurant_order"."date" IS NULL THEN "restaurant_order"."id" ELSE NULL END) AS "cnt" FROM "restaurant_table" LEFT OUTER JOIN "restaurant_order" ON ("restaurant_table"."id" = "restaurant_order"."table_id") GROUP BY "restaurant_table"."id", "restaurant_table"."hall_id", "restaurant_table"."sha
pe", "restaurant_table"."number", "restaurant_table"."seats", "restaurant_table"."width", "restaurant_table"."height", "restaurant_table"."coordinate_x", "restaurant_table"."coordinate_y", "restaurant_table"."is_reserved" HAVING COUNT(CASE WHEN ("restaurant_order"."date" IS NULL) THEN "restaurant_order"."id"
 ELSE NULL END) > 0