对Django Q对象执行逻辑异或

Question

我想在django.db.models.Q对象上执行逻辑异或(XOR),使用运算符模块将模型字段的选择限制为外键的子集.我在Django 1.4.3和Python 2.7.2中这样做.我有这样的事情:

import operator

from django.conf import settings
from django.db import models
from django.db.models import Q
from django.contrib.auth.models import User, Group

def query_group_lkup(group_name):
    return Q(user__user__groups__name__exact=group_name)

class Book(models.Model):
    author = models.ForeignKey(
                 User,
                 verbose_name=_("Author"),
                 null=False,
                 default='',
                 related_name="%(app_label)s_%(class)s_author",
                 # This would have provide an exclusive OR on the selected group name for User
                 limit_choices_to=reduce(
                     operator.xor,
                     map(query_group_lkup, getattr(settings, 'AUTHORIZED_AUTHORS', ''))
                 )

AUTHORIZED_AUTHORS 是现有组名称的列表.

但这不起作用,因为Q对象不支持^运算符(仅来自文档的 |和&运算符).来自stacktrace的消息(部分)如下:

File "/home/moi/.virtualenvs/venv/lib/python2.7/site-packages/django/db/models/loading.py", line 64, in _populate
    self.load_app(app_name, True)
  File "/home/moi/.virtualenvs/venv/lib/python2.7/site-packages/django/db/models/loading.py", line 88, in load_app
    models = import_module('.models', app_name)
  File "/home/moi/.virtualenvs/venv/lib/python2.7/site-packages/django/utils/importlib.py", line 35, in import_module
    __import__(name)
  File "/opt/dvpt/toto/apps/book/models.py", line 42, in <module>
    class Book(models.Model):
  File "/opt/dvpt/toto/apps/book/models.py", line 100, in Book
    map(query_group_lkup, getattr(settings, 'AUTHORIZED_AUTHORS', ''))
TypeError: unsupported operand type(s) for ^: 'Q' and 'Q'

因此,受此答案的启发,我试图为我的特定查找实现XOR.它不是很灵活,因为查找是硬编码的(我需要在query_xor的参数中使用kwargs,例如......).我最终做了这样的事情:

from django.conf import settings
from django.db import models
from django.db.models import Q
from django.db.models.query import EmptyQuerySet
from django.contrib.auth.models import User, Group

def query_xor_group(names_group):
    """Get a XOR of the queries that match the group names in names_group."""

    if not len(names_group):
        return EmptyQuerySet()
    elif len(names_group) == 1:
        return Q(user__user__groups__name__exact=names_group[0])

    q_chain_or = Q(user__user__groups__name__exact=names_group[0])
    q_chain_and = Q(user__user__groups__name__exact=names_group[0])

    for name in names_group[1:]:
        query = Q(user__user__groups__name__exact=name)
        q_chain_or |= query
        q_chain_and &= query

    return q_chain_or & ~q_chain_and

class Book(models.Model):
    author = models.ForeignKey(
                 User,
                 verbose_name=_("author"),
                 null=False,
                 default='',
                 related_name="%(app_label)s_%(class)s_author",
                 # This provides an exclusive OR on the SELECT group name for User
                 limit_choices_to=query_xor_group(getattr(settings, 'AUTHORIZED_AUTHORS', ''))
                 )

它按我的意思工作,但我觉得我不是pythonic(尤其是query_xor_group方法).这样做会有更好的(更直接的方式)吗？

基本上,我的问题可以被剥夺limit_choices_to部分,并总结为:

如何以Djangonic方式对一组django.db.models.Q对象进行按位异或？

Answer 1

Django 4.1添加了对 XOR 的支持：

Q现在可以使用^异或 ( XOR) 运算符来组合对象和查询集。XORMariaDB 和 MySQL 本身支持。对于不支持的数据库XOR，查询将转换为使用AND、OR和 的等效查询NOT。

这意味着您现在无需猴子修补即可编写Foobar.objects.filter(Q(blah=1) ^ Q(bar=2))。
九年的等待是值得的，不是吗？

Answer 2

您可以__xor__()向使用和/或/不执行XOR逻辑的Q 添加方法.

from django.db.models import Q

class QQ:
    def __xor__(self, other):    
        not_self = self.clone()
        not_other = other.clone()
        not_self.negate()
        not_other.negate()

        x = self & not_other
        y = not_self & other

        return x | y

Q.__bases__ += (QQ, )

这样做后,我能够Q(...) ^ Q(...)在filter()通话.

Foobar.objects.filter(Q(blah=1) ^ Q(bar=2)) 

这意味着原始尝试不再抛出不受支持的操作数异常.

limit_choices_to=reduce(
                     operator.xor,
                     map(query_group_lkup, getattr(settings, 'AUTHORIZED_AUTHORS', ''))
                 )

经测试,在Django 1.6.1上Python 2.7.5