Question

我想在django.db.models.Q对象上执行逻辑异或（XOR），使用operator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是现有群组名称的列表。

但这不起作用，因为Q对象不支持^运算符（docs只有|和＆amp;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answer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，我的问题可能会被剥夺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

您可以向使用和/或/不执行XOR逻辑的Q添加__xor__()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, )

执行此操作后，我可以Q(...) ^ Q(...)拨打filter()。

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', ''))
                 )

在Django 1.6.1

上Python 2.7.5进行了测试

对Django Q对象执行逻辑异或

1 个答案: