Django Database querying differences

Question

I am going through the creation of the Polls app again, in the Django Docs. I wanted to ask again about one particular thing they do in the django database. The code is shown below:

>>> 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?>

If you take a look at the variable c, you will see that it is created using this, p.choice_set.create(choice_text='Just hacking again', votes=0). Now if you created it instead with this: c = p.choice_set.filter(id=3), and when you type in c.poll, it will give you an error. Why does this happen? The console gives me this error : AttributeError: 'Poll' object has no attribute 'create', but I do not understand what it means.

Also, is there any way of getting c.poll to give you an output without having to create a new choice?

-- Thanks in advance

Answer 1

c = p.choice_set.filter(id=3) won't return a single choice object. It returns a queryset composed of a single choice object because, obviously, there is just one object with the same id. Querysets are iterables, which means that if you want to obtain the choice object from that variable it should be: c = p.choice_set.filter(id=3)[0] That is the difference with choice_set.create: create returns the single created object.

Now, that's not the way to do it. When you know you are querying for a single object, use get. c = p.choice_set.get(id=3).

See querying documentation for further details.