Django数据库查询差异

Question

我正在通过在Django Docs中再次创建投票应用程序。我想再问一次他们在django数据库中做的一件事。代码如下所示：

>>> from polls.models import Poll, Choice 

# Make sure our __unicode__() addition worked. 
>>> Poll.objects.all() 
[<Poll: What's up?>] 

# Django provides a rich database lookup API that's entirely driven by 
# keyword arguments. 
>>> Poll.objects.filter(id=1) 
[<Poll: What's up?>] 
>>> Poll.objects.filter(question__startswith='What') 
[<Poll: What's up?>] 

# Get the poll that was published this year. 
>>> from django.utils import timezone 
>>> current_year = timezone.now().year 
>>> Poll.objects.get(pub_date__year=current_year) 
<Poll: What's up?> 

# Request an ID that doesn't exist, this will raise an exception. 
>>> Poll.objects.get(id=2) 
Traceback (most recent call last): 
    ... 
DoesNotExist: Poll matching query does not exist. Lookup parameters were {'id': 2} 

# Lookup by a primary key is the most common case, so Django provides a 
# shortcut for primary-key exact lookups. 
# The following is identical to Poll.objects.get(id=1). 
>>> Poll.objects.get(pk=1) 
<Poll: What's up?> 

# Make sure our custom method worked. 
>>> p = Poll.objects.get(pk=1) 
>>> p.was_published_recently() 
True 

# Give the Poll a couple of Choices. The create call constructs a new 
# Choice object, does the INSERT statement, adds the choice to the set 
# of available choices and returns the new Choice object. Django creates 
# a set to hold the "other side" of a ForeignKey relation 
# (e.g. a poll's choices) which can be accessed via the API. 
>>> p = Poll.objects.get(pk=1) 

# Display any choices from the related object set -- none so far. 
>>> p.choice_set.all() 
[] 

# Create three choices. 
>>> p.choice_set.create(choice_text='Not much', votes=0) 
<Choice: Not much> 
>>> p.choice_set.create(choice_text='The sky', votes=0) 
<Choice: The sky> 
>>> c = p.choice_set.create(choice_text='Just hacking again', votes=0) 

# Choice objects have API access to their related Poll objects. 
>>> c.poll 
<Poll: What's up?> 

如果你看一看变量c，你会看到它是用这个，p.choice_set.create(choice_text='Just hacking again', votes=0)创建。现在，如果您创建它，而不是这样：c = p.choice_set.filter(id=3)，并且当您输入c.poll时，它会给您一个错误。为什么会发生？控制台给了我这个错误：AttributeError: 'Poll' object has no attribute 'create'，但我不明白它的意思。

此外，有没有让c.poll给你，而无需创建一个新的选择输出的方法吗？

- 预先感谢

Answer 1

c = p.choice_set.filter(id=3)不会返回一个选择对象。它返回由单个选择对象组成的查询集，因为显然只有一个具有相同ID的对象。查询集是iterables，这意味着如果你想获得该变量的选择对象应该是：c = p.choice_set.filter(id=3)[0] 这是choice_set.create的区别是：创造回报单创建的对象。

现在，这不是做到这一点的方法。当你知道你正在查询单个对象时，使用get。 c = p.choice_set.get(id=3)。

进一步详情，请参阅querying documentation。