Django: 使用 Q 对象构建复杂的查询语句

      本文将讲述如何在 Django 项目中使用 Q 对象构建复杂的查询条件。 假设有如下的 model:

class Question(models.Model):
    question_text = models.CharField(max_length=200)
    pub_date = models.DateTimeField('date published')

然后我们创建了一些数据：

Question.objects.create(
    question_text='what are you doing',
    pub_date=datetime.datetime(2015, 11,7)
)
Question.objects.create(
    question_text='what is wrong with you',
    pub_date=datetime.datetime(2014, 11, 7)
)
Question.objects.create(
    question_text='who are you',
    pub_date=datetime.datetime(2015, 10, 7)
)
Question.objects.create(
    question_text='who am i',
    pub_date=datetime.datetime(2014, 10, 7)
)

>>> Question.objects.all()
[<Question: what are you doing>, <Question: what is wrong with you>,
 <Question: who are you>, <Question: who am i>]

AND 查询

将多个 Q 对象作为非关键参数或使用 & 联结即可实现 AND 查询:

>>> from django.db.models import Q
# Q(...)
>>> Question.objects.filter(Q(question_text__contains='you'))
[<Question: what are you doing>, <Question: what is wrong with you>, <Question: who are you>]

# Q(...), Q(...)
>>> Question.objects.filter(Q(question_text__contains='you'), Q(question_text__contains='what'))
[<Question: what are you doing>, <Question: what is wrong with you>]

# Q(...) & Q(...)
>>> Question.objects.filter(Q(question_text__contains='you') & Q(question_text__contains='what'))
[<Question: what are you doing>, <Question: what is wrong with you>]

OR 查询

使用 | 联结两个 Q 对象即可实现 OR 查询:

# Q(...) | Q(...)
>>> Question.objects.filter(Q(question_text__contains='you') | Q(question_text__contains='who'))
[<Question: what are you doing>, <Question: what is wrong with you>, <Question: who are you>, <Question: who am i>]

NOT 查询

使用 ~Q(...) 客户实现 NOT 查询:

# ~Q(...)
>>> Question.objects.filter(~Q(question_text__contains='you'))
[<Question: who am i>]

与关键字参数共用

记得要把 Q 对象放前面:

# Q(...), key=value
>>> Question.objects.filter(Q(question_text__contains='you'), question_text__contains='who')
[<Question: who are you>]

OR, AND, NOT 多条件查询

这几个条件可以自由组合使用:

# (A OR B) AND C AND (NOT D)
>>> Question.objects.filter((Q(question_text__contains='you') | Q(question_text__contains='who')) & Q(question_text__contains='what') & ~Q(question_text__contains='are'))
[<Question: what is wrong with you>]

动态构建查询条件

比如你定义了一个包含一些 Q 对象的列表，如何使用这个列表构建 AND 或 OR 查询呢？ 可以使用 operator 和 reduce：

>>> lst = [Q(question_text__contains='you'), Q(question_text__contains='who')]

# OR
>>> Question.objects.filter(reduce(operator.or_, lst))
[<Question: what are you doing>, <Question: what is wrong with you>, <Question: who are you>, <Question: who am i>]

# AND
>>> Question.objects.filter(reduce(operator.and_, lst))
[<Question: who are you>]

这个列表也可能是根据用户的输入来构建的，比如简单的搜索功能（搜索一个文章的标题或内容或作者名称包含某个关键字）:

q = request.GET.get('q', '').strip()
lst = []
if q:
    for key in ['title__contains', 'content__contains',
                'author__name__contains']:
        q_obj = Q(**{key: q})
        lst.append(q_obj)
queryset = Entry.objects.filter(reduce(operator.or_, lst))

参考资料

• Parerga und Paralipomena » Blog Archive » The power of django’s Q objects
• Making queries | Django documentation | Django
• QuerySet API reference | Django documentation | Django
• django/tests.py at master · django/django · GitHub
• 9.9. operator — Standard operators as functions — Python 2.7.10 documentation
• 2. Built-in Functions — Python 2.7.10 documentation