يقوم Django بإنشاء استعلام فرعي في عبارة group-by الخاصة باستعلام SQL

لدي النماذج التالية

class Task(models.Model):
    project = models.ForeignKey(Project, on_delete=models.CASCADE)
    author = models.ForeignKey(User, on_delete=models.CASCADE)

class Project(models.Model):
    name = models.CharField(max_length=128)

class EmployeeProject(models.Model):
    project = models.ForeignKey(
        Project,
        on_delete=models.CASCADE,
        related_name='employee_projects',
    )
    employee = models.ForeignKey(Employee, on_delete=models.CASCADE)
    role = models.ForeignKey(ProjectRole, on_delete=models.CASCADE)
    skill_level = models.ForeignKey(SkillLevel, on_delete=models.CASCADE)
    hourly_rate = models.DecimalField(decimal_places=2, max_digits=9)

class TaskRole(LogOnSaveMixin, DiffModel):
    task = models.ForeignKey(
       Task, on_delete=models.CASCADE,
       related_name='roles',
    )
    estimated_hours = models.FloatField(null=True, blank=True)

الاستعلامات

qs = Task.objects.all()


user = self.request.user
host_roles = EmployeeProject.objects.filter(
      employee=user.employee,
)
qs = qs.annotate(
    total_estimated_hours=Sum('roles__estimated_hours'),
)
open_tasks_qs = qs.filter(
   Q(project__in=host_roles.values('project_id')),
 ).filter(
            (
                Q(total_estimated_hours__isnull=False)
            ),
        )

qs = open_tasks_qs | qs.filter(author=user).distinct()
qs = qs.values('id')
qs = qs.distinct()
return qs

المشكلة هي أن هذا الاستعلام ينتج عنه خطأ 

more than one row returned by a subquery used as an expression

يبدو SQL المقابل كما يلي:

SELECT 
  DISTINCT "tasks_task"."id" 
FROM 
  "tasks_task" 
  LEFT OUTER JOIN "tasks_taskrole" ON (
    "tasks_task"."id" = "tasks_taskrole"."task_id"
  ) 
GROUP BY 
  "tasks_task"."id", 
  (
    SELECT 
      U0."project_id" 
    FROM 
      "tasks_employeeproject" U0 
    WHERE 
      U0."employee_id" = 1
  ) 
HAVING 
  (
    (
      "tasks_task"."project_id" IN (
        SELECT 
          U0."project_id" 
        FROM 
          "tasks_employeeproject" U0 
        WHERE 
          U0."employee_id" = 1
      ) 
      AND SUM(
        "tasks_taskrole"."estimated_hours"
      ) IS NOT NULL
    ) 
    OR "tasks_task"."author_id" = 1
  ) 
ORDER BY 
  "tasks_task"."id" DESC

هناك سؤالان:
كيف تصلح المشكلة؟
كيف يمكن أن يكون استعلام فرعي جزءا من عبارة group by؟ حاولت أن أجد وثائق SQL على جوجل ، لكنني لم أجد شيئا