第二课¶

设置数据库¶

在mysite/setting.py中的Databases设置相关参数

支持的数据库

PostgreSQL，需要 psycopg2，详细参考 PostgreSQL notes
MySQL，需要 DB API driver 例如：mysqlclient。详情参考 MySQL backend注意事项
SQLite，参考 SQLite backend注意事项
Oracle，需要 cx_Oracle，详细参考 Oracle backend注意事项

修改 DATABASE ‘default’ 可以改变数据库的类型。

ENGINE-支持 ‘django.db.backends.sqlite3’ , ‘django.db.backends.postgresql’ , ‘django.db.backends.mysql’ ,或者 ‘django.db.backends.oracle’
NAME-数据库名字。

还需要设置：USER, PASSWORD, HOST

确保自己的app在 INSTALLED_APPS

初始化数据库表:

$ python manage.py migrate

创建Models¶

在polls应用中创建2个models， Question 和 Choice 。一个 Question 有一个question和一个发布日期。一个 Choice 有两个字段，选择项文本和一个投票计数，每个 Choice 可以关联一个 Question 。

polls/models.py

from django.db import models

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


class Choice(models.Model):
    question = models.ForeignKey(Question, on_delete=models.CASCADE)
    choice_text = models.CharField(max_length=200)
    votes = models.IntegerField(default=0)

激活Models¶

需要将apps加入到project中来，需要在setting文件中的 INSTALLED_APPS 里指明。这里PollsConfig类是在 polls/apps.py 里，所以路劲是 polls.apps.PollsConfig 。

mysite/settings.py

INSTALLED_APPS = [
  'polls.apps.PollsConfig',
  'django.contrib.admin',
  'django.contrib.auth',
  'django.contrib.contenttypes',
  'django.contrib.sessions',
  'django.contrib.messages',
  'django.contrib.staticfiles',
  ]

现在Django知道包含了 polls 应用，运行如下命令:

$ python manage.py makemigrations polls

通过 makemigrations 告诉Django，你已经修改了models。

运行下面的命令能看到models修改的sql过程:

$ python manage.py sqlmigrate polls 0001

Playing with the API¶

使用以下命令进入Django命令行交互模式:

$ python manage.py shell

使用这个命令代替”python”的原因是 manage.py 能设置 DJANGO_SETTING_MODULE 环境变量。

开始探索 database API

# 导入我们刚刚创建的model。
>>> from polls.models import Question, Choice

# 现在还没有Question在我们的数据库里。
>>> Question.objects.all()
<QuerySet []>

# 创建一个新的Question。
# 默认settings文件里已经支持了时区，
# 因此Django希望pub_date是带有时区的时间格式。
# 用timezone.now()代替datetime.datetime.now()
>>> from django.utils import timezone
>>> q = Question(question_text="What's new?", pub_date=timezone.now())

# 保存这个object进入数据库，你只需使用save()
>>> q.save()

# 现在它有了个ID。注意这里有可能用"1L"来代替"1"，这取决于你的正在使用的数据库。
>>> q.id
1

# 通过Python的属性来访问model的字段
>>> q.question_text
"What's new?"
>>> q.pub_date
datetime.datetime(2012, 2, 26, 13, 0, 0, 775217, tzinfo=<UTC>)

# 通过改变属性来更改值，然后使用save()
>>> q.question_text = "What's up?"
>>> q.save()

# object.all()会显示所有数据库里的数据。
>>> Question.objects.all()
<QuerySet [<Question: Question object>]>

<Question: Question object> 看起来不怎么方便。修改 Question model（在 polls/models.py ）并且增加一个 __str__ 方法就显示的更加友好。

polls/models.py:

from django.db import models
from django.utils.encoding import python_2_unicode_compatible

@python_2_unicode_compatible  # only if you need to support Python 2
class Question(models.Model):
    # ...
    def __str__(self):
        return self.question_text

@python_2_unicode_compatible  # only if you need to support Python 2
class Choice(models.Model):
    # ...
    def __str__(self):
        return self.choice_text

这是一个常规的Python方法，我们来自建一个方法

polls/models.py:

import datetime

from django.db import models
from django.utils import timezone


class Question(models.Model):
    # ...
    def was_published_recently(self):
        return self.pub_date >= timezone.now() - datetime.timedelta(days=1)

保存，通过 python manage.py shell 重新打开一个Django命令行:

>>> from polls.models import Question, Choice

# 确认一下__str__()是否工作正常。
>>> Question.objects.all()
<QuerySet [<Question: What's up?>]>

# Django提供了丰富的数据库查询API，通过关键字来查询。
>>> Question.objects.filter(id=1)
<QuerySet [<Question: What's up?>]>
>>> Question.objects.filter(question_text__startswith='What')
<QuerySet [<Question: What's up?>]>

# 获取一个今年发布的Question
>>> from django.utils import timezone
>>> current_year = timezone.now().year
>>> Question.objects.get(pub_date__year=current_year)
<Question: What's up?>

# 回应的ID不存在，会抛出一个错误
>>> Question.objects.get(id=2)
Traceback (most recent call last):
    ...
DoesNotExist: Question matching query does not exist.

# 通过主键来查询是一件平常事，
# 因此Django提供了主键查询的快捷方式
# 下面等价于Question.objects.get(id=1)
>>> Question.objects.get(pk=1)
<Question: What's up?>

# 确认一下自定义方法是否工作正常
>>> q = Question.objects.get(pk=1)
>>> q.was_published_recently()
True

# Give the Question 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 question's choice) which can be accessed via the API.
# TODO: 这段不是很理解。
>>> q = Question.objects.get(pk=1)

# 显示所有相关的Choices
>>> q.choice_set.all()
<QuerySet []>

# 新建3个Choices
>>> q.choice_set.create(choice_text='Not much', votes=0)
<Choice: Not much>
>>> q.choice_set.create(choice_text='The sky', votes=0)
<Choice: The sky>
>>> c = q.choice_set.create(choice_text='Just hacking again', votes=0)

# Choice object可以通过API来访问与它关联的Question objects
>>> c.question
<Question: What's up?>

# And vice versa: Question objects get access to Choice objects.
>>> q.choice_set.all()
<QuerySet [<Choice: Not much>, <Choice: The sky>, <Choice: Just hacking again>]>
>>> q.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 question whose pub_date is in this year
# (reusing the 'current_year' variable we created above).
>>> Choice.objects.filter(question__pub_date__year=current_year)
<QuerySet [<Choice: Not much>, <Choice: The sky>, <Choice: Just hacking again>]>

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