Django for ISchoolers: Difference between revisions

<code>

>>> from qandabear.models import Question, Answer</code>

<code>

>>> Question.objects.all()<br>

<code>

# We could specify the value for pub_date as well here, but we don't have to since when we wrote its model definition, we specified that pub_date has a default value (of when the question is created)</code>

* Try getting the ID number of our new question 'q' by typing <code>q.id</code>. What happens?

<code>

>>> q.save()</code>

* Get the id of the Question instance. Because it’s been saved, it has an ID in the database now!

<code>

>>> q.id

<code>

>>> q.text

"What is the Weirdest Cookbook Ever?"

>>> q.pub_date

datetime.datetime(2011, 12, 1, 3, 3, 55, 841929)

</code>

>>> import datetime<br>

>>> q.pub_date = datetime.datetime(2011, 4, 1, 0, 0)<br>

# return the answer text, or if the text is longer than 37 characters, return a summary with an ellipsis

summary = self.text[:37]

if len(self.text) > 37:

It’s important to add __unicode__() methods to your models, not only for your own sanity when dealing with the interactive prompt, but also because if you use Django's automatically-generated admin, these representations are used there too.

Enough of these normal python methods! Let's build in some useful functionality to our models in qandabear/models.py.

Note the addition of <code>import datetime</code> to reference Python’s standard datetime module. This allows us to use the datetime library module in models.py by calling it with datetime.

<code>

>>> from qandabear.models import Question, Answer

>>> Question.objects.all()

[<Question: What is the Weirdest Cookbook Ever?>]

Search your database using the filter method on the objects attribute of Question.

>>> questions = Question.objects.filter(text="What is the Weirdest Cookbook Ever?")

>>> questions

>>> questions[0].id # remember python lists start with element 0.

</code>

If you try to search for a question that does not exist, filter will give you the empty list. The get method will always return one hit, or raise an exception.

<code>

>>> Question.objects.filter(text="Who framed Roger Rabbit?")

[]

>>> Question.objects.get(id=1)

<Question: What is the Weirdest Cookbook Ever?>

>>> Question.objects.get(id=2)

DoesNotExist: Question matching query does not exist.

</code>

===Add Answers===

Observe: there is a Question in the database, but it has no Answers! Let's fix that.

<code>

>>> q = Question.objects.get(id=1)

>>> q.answer_set.all()

Create three answers:

>>> q.answer_set.create(text='To Serve Man')

<Answer: To Serve Man>

>>> q.answer_set.create(text='The Original Road Kill Cookbook')

<Answer: The Original Road Kill Cookbook>

>>> a = q.answer_set.create(text='Mini-Mart A La Carte')

>>> a

<Answer: Mini-Mart A La Carte>

</code>

Go in reverse! Find the question a particular answer belongs to:

<code>

>>> a.question

<Question: What is the Weirdest Cookbook Ever?>

</code>

Because a Question can have more than one Answer, Django creates the answer_set attribute on each Question. You can use that to look at the list of available Answer, or to create them.

<code>

>>> q.answer_set.all()

[<Answer: To Serve Man>, <Answer: The Original Road Kill Cookbook>, <Answer: Mini-Mart A La Carte>]

>>> q.answer_set.count()

</code>

Can one be a Answer for a Question that doesn’t yet exist?:

<code>

>>> koan = Answer("Is this even an answer")

>>> koan.question_id

>>> koan.question

</code>

<!--- Adding pre-written data would go here. -->

=== Changing your models ===

Oh no! Our client, LulzTech, has decided to change the spec for this prototype. Two changes, in fact. First, they've decided that tracking the pub_date of Questions is silly, and they want to remove that field. Second, they want to add a votes field to each Answer, to track how many upvotes it has. Which means we’re going to have to change our models.

* Open qandabear/models.py and edit the Question class:

<code>

class Question(models.Model):

question = models.CharField(max_length=200)

</code>

* Edit the Answer class too:

<code>

class Answer(models.Model):

question = models.ForeignKey(Question)

text = models.TextField()

votes = models.IntegerField(default=0)

pub_date = models.DateTimeField(auto_now_add=True)

</code>

The default parameter lets us set a default value for this field if an answer's vote count isn't explicitly specified. Most new answers are going to have zero votes, so we set our default to 0.

* Make a migration so the database knows we deleted Question.pub_date and added Answer.votes:

<code>

$ python manage.py schemamigration qandabear --auto

</code>

* Apply the migration.

<code>

$ python manage.py migrate qandabear

</code>

===Save and commit===

You know the drill!