获取 Django 中对象的后代

Question

我想编写一个方法，将对象的所有后代(即子代、孙子、曾孙类......(作为 QuerySet 返回。

我写了两个方法，第一种(get_all_children(可以返回子孙，但如果有曾孙(或下后代(，则会出现以下错误：

django.db.utils.ProgrammingError: (1064, "You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'UNION (SELECT DISTINCT `deneme_app_person`.`id`, `deneme_app_person`.`name`, `de' at line 1")

第二种方法(get_all_children_list(工作正常(据我测试(，但可能更慢，最后一行看起来不太好。

models.py

from django.db import models

# Create your models here.
class Person(models.Model):
name = models.CharField(max_length=100)
parent = models.ForeignKey(
'self', null=True, blank=True, related_name='children',
on_delete=models.CASCADE)
def get_all_children(self):
"""
This method will return all children and grandchildren of Person objects
:return:
"""
p_list = self.children.all()
if p_list.count():
for p in p_list:
children = p.get_all_children()
if children.count():
p_list = p_list.union(children)
return p_list.distinct()
def get_all_children_list(self):
p_list = self.children.all()
if p_list.count():
for p in p_list:
children = p.get_all_children_list()
if children.count():
p_list = set(p_list).union(children)
# p_list = list(set(p_list + children))
return Person.objects.filter(id__in=[p.id for p in p_list])
# return list(p_list)
def __str__(self):
return f"({self.id}) - {self.name}"

tests.py

from django.test import TestCase
from django.db.models.query import QuerySet
from .models import Person

class PersonTests(TestCase):
def test_get_all_children(self):
p = Person.objects.create(name='Jack')
p1 = Person.objects.create(name='Jack Jr', parent=p)
p2 = Person.objects.create(name='Jill', parent=p)
p11 = Person.objects.create(name='Jack Jr2', parent=p1)
p111 = Person.objects.create(name='Jack Jr3', parent=p11)
p112 = Person.objects.create(name='Jill Jr', parent=p11)
# Following test is working
self.assertEqual([p11, p111, p112], list(p1.get_all_children()))
# Following test is not working
self.assertEqual([p1, p2, p11, p111, p112], list(p.get_all_children()))
for person in [p, p1, p2, p11, p111, p112]:
self.assertIsInstance(person.get_all_children(), QuerySet)
def test_get_all_children_list(self):
p = Person.objects.create(name='Jack')
p1 = Person.objects.create(name='Jack Jr', parent=p)
p2 = Person.objects.create(name='Jill', parent=p)
p11 = Person.objects.create(name='Jack Jr2', parent=p1)
p111 = Person.objects.create(name='Jack Jr3', parent=p11)
p112 = Person.objects.create(name='Jill Jr', parent=p11)
p1111 = Person.objects.create(name='Jack Jr4', parent=p111)
self.assertEqual([p1111], list(p111.get_all_children_list()))
self.assertEqual([p111, p112, p1111], list(p11.get_all_children_list()))
self.assertEqual([p11, p111, p112, p1111], list(p1.get_all_children_list()))
self.assertEqual([p1, p2, p11, p111, p112, p1111], list(p.get_all_children_list()))
for person in [p, p1, p2, p11, p111, p112, p1111]:
self.assertIsInstance(person.get_all_children_list(), QuerySet)

你能指出第一种方法有什么问题吗？如何纠正这种方法？

Answer 1

我已经在本地重现了您的错误，但我还没有弄清楚背后的原因。 如果你不介意不使用union，你可以使用|运算符来合并两个查询集，所以你的方法将变成这样：

def get_all_children(self):
children = self.children.all()
for child in children:
children = children | child.get_all_children()
return children