【Django tutorial 2】

5.创建应用

    在之前创建的项目(pollsite)中创建应用(polls)，方法很简单。进入pollsite文件夹执行以下命令：

python manage.py startapp polls

    这条命令创建了一个名为polls的文件夹，内容如下：

polls/
    __init__.py
    models.py
    tests.py
    views.py

6.为应用的数据库定义数据模型

    在我们的投票系统中，包含两个数据模型：polls、choices，它们的具体定义需要在polls/models.py中完成：

from django.db import models

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

class Choice(models.Model):
    poll = models.ForeignKey(Poll)
    choice = models.CharField(max_length=200)
    votes = models.IntegerField()

这些代码很明显的定义了poll和choice两个模型，其中question和pub_date是模型中的字段名称，同时也是数据库表中的列名。其中max_length既是数据库中的字段约束，同时也是验证方法。

7. 激活数据模型

我们可以使用上述定义的数据模型来创建对应的数据库表，并且创建一个可以通过Python访问的数据库API。

首先在settings.py中的INSTALLED_APPS里，增加polls

INSTALLED_APPS = (
    'django.contrib.auth',
    'django.contrib.contenttypes',
    'django.contrib.sessions',
    'django.contrib.sites',
    'polls'
)

执行命令：

python manage.py sql polls

接着可以看到如下类似内容：

BEGIN;
CREATE TABLE "polls_poll" (
    "id" serial NOT NULL PRIMARY KEY,
    "question" varchar(200) NOT NULL,
    "pub_date" timestamp with time zone NOT NULL
);
CREATE TABLE "polls_choice" (
    "id" serial NOT NULL PRIMARY KEY,
    "poll_id" integer NOT NULL REFERENCES "polls_poll" ("id"),
    "choice" varchar(200) NOT NULL,
    "votes" integer NOT NULL
);
COMMIT;

这其中表的名字以及表中字段的名字都是自动生成的。命令中的sql并不是在数据库中真正执行SQL命令，仅仅是告诉你Django需要的数据库内容。真正在数据库中创建表需要执行如下命令：

python manage.py syncdb

syncdb用来创建那些在数据库中不存在的表格，言外之意就是与模型描述同步。这条命令应该频繁使用以保证数据库与模型之间相同步。

8.通过python shell与Django进行交互

python manage.py shell

>>> from polls.models import Poll, Choice # Import the model classes we just wrote.

# No polls are in the system yet.
>>> Poll.objects.all()
[]

# Create a new Poll.
>>> import datetime
>>> p = Poll(question="What's up?", pub_date=datetime.datetime.now())

# Save the object into the database. You have to call save() explicitly.
>>> p.save()

# Now it has an ID. Note that this might say "1L" instead of "1", depending
# on which database you're using. That's no biggie; it just means your
# database backend prefers to return integers as Python long integer
# objects.
>>> p.id
1

# Access database columns via Python attributes.
>>> p.question
"What's up?"
>>> p.pub_date
datetime.datetime(2007, 7, 15, 12, 00, 53)

# Change values by changing the attributes, then calling save().
>>> p.pub_date = datetime.datetime(2007, 4, 1, 0, 0)
>>> p.save()

# objects.all() displays all the polls in the database.
>>> Poll.objects.all()
[<Poll: Poll object>]

增加两个模型可unicode的方法：

class Poll(models.Model):
    # ...
    def __unicode__(self):
        return self.question

class Choice(models.Model):
    # ...
    def __unicode__(self):
        return self.choice

接着为Poll增加一个自定义的方法：

import datetime
# ...
class Poll(models.Model):
    # ...
    def was_published_today(self):
        return self.pub_date.date() == datetime.date.today()

再次使用Python Shell进行交互，（命令：python manage.py shell）

>>> 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 whose year is 2007.
>>> Poll.objects.get(pub_date__year=2007)
<Poll: What's up?>

>>> Poll.objects.get(id=2)
Traceback (most recent call last):
    ...
DoesNotExist: Poll matching query does not exist.

# 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_today()
False

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

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

# And vice versa: Poll objects get access to Choice objects.
>>> p.choice_set.all()
[<Choice: Not much>, <Choice: The sky>, <Choice: Just hacking again>]
>>> p.choice_set.count()
3

# The API automatically follows relationships as far as you need.
# Use double underscores to separate relationships.
# This works as many levels deep as you want; there's no limit.
# Find all Choices for any poll whose pub_date is in 2007.
>>> Choice.objects.filter(poll__pub_date__year=2007)
[<Choice: Not much>, <Choice: The sky>, <Choice: Just hacking again>]

# Let's delete one of the choices. Use delete() for that.
>>> c = p.choice_set.filter(choice__startswith='Just hacking')
>>> c.delete()

请继续阅读tutorial3