django开发5

Django 05

Django数据模型层使用了ORM(对象关系映射).我们以操作python的对象的方式去访问数据库。

定义数据表

在django中, 我们不需要直接在数据库中定义数据表(CREATE TABLE), 而是在models.py文件中定义一个继承Model的类, 接下来在这个类中定义一系列Field类型的类属性来描述数据的每个字段.

from django.db import models

class Blog(models.Model):
    title = models.CharField(max_length=150)
    body  = models.TextField()
    timestamp = models.DateTimeField()

定义好数据表后, 我们使用django提供的工具, 来由这些Model的子类来创建数据库中的表.
1. 首先要确保, 这个应用在settings中的INSTALLED_APPS中.

INSTALLED_APPS = [
    'django.contrib.admin',
    'django.contrib.auth',
    'django.contrib.contenttypes',
    'django.contrib.sessions',
    'django.contrib.messages',
    'django.contrib.staticfiles',
    # my app
    'blog',
]

1. 使用makemigrations命令, 来告诉django我们的model的做了改动.

$ python manage.py makemigrations blog

Migrations for 'blog':
  blog/migrations/0001_initial.py:
    - Create model Blog

此时, 数据库还未发生真正的改动, 我们可以使用sqlmigrate命令, 看看我们修改model, 对应的修改数据库的SQL语句.(此步骤可选)

$ python manage.py sqlmigrate blog 0001

BEGIN;
--
-- Create model Blog
--
CREATE TABLE "blog_blog" (
"id" integer NOT NULL PRIMARY KEY AUTOINCREMENT, 
"title" varchar(150) NOT NULL, 
"body" text NOT NULL, 
"timestamp" datetime NOT NULL);
COMMIT;

1. 这时我们使用migrate, 改变数据库.

$ python manage.py migrate

Operations to perform:
  Apply all migrations: admin, auth, blog, contenttypes, sessions
Running migrations:
  Applying contenttypes.0001_initial... OK
  Applying auth.0001_initial... OK
  Applying admin.0001_initial... OK
  Applying admin.0002_logentry_remove_auto_add... OK
  Applying contenttypes.0002_remove_content_type_name... OK
  Applying auth.0002_alter_permission_name_max_length... OK
  Applying auth.0003_alter_user_email_max_length... OK
  Applying auth.0004_alter_user_username_opts... OK
  Applying auth.0005_alter_user_last_login_null... OK
  Applying auth.0006_require_contenttypes_0002... OK
  Applying auth.0007_alter_validators_add_error_messages... OK
  Applying auth.0008_alter_user_username_max_length... OK
  Applying blog.0001_initial... OK
  Applying sessions.0001_initial... OK

之后每次新建或更新model, 我们还要执行makemigrations和migrate这俩条命令更新数据库.

常用Field

• CharField: 字符串(varchar)
• EmailField: 邮件地址(varchar)
• IPAddressField: IP地址(char)

CharField(max_length=64)
CharField(max_length=32, blank=True)

• TextField: 文本(text)
• IntegerField: 整数(integer)

IntegerField()
IntegerField(default=0)

• FloatField: 浮点数(real)
• BooleanField: 布尔(bool)
• TimeField: 时间(time)
• DateField: 日期(date)
• DateTimeField: 日期时间(datetime)
• FileField: 文件
• ImageField: 图片

多对一关系

使用ForeignKey来描述多对一关系, 例如一个作者写了多本书, 也就是多本书对应一个作者.

class Book(models.Model):
    title = models.CharField(max_length=256)
    author = models.ForeignKey('Author')
    price = models.FloatField()

class Author(models.Model):
    name = models.CharField(max_length=256)
    age = models.IntegerField()

根据上面的model会创建出如下的表.

--
-- Create model Author
--
CREATE TABLE "blog_author" (
    "id" integer NOT NULL PRIMARY KEY AUTOINCREMENT, 
    "name" varchar(256) NOT NULL, 
    "age" integer NOT NULL
);
--
-- Create model Book
--
CREATE TABLE "blog_book" (
    "id" integer NOT NULL PRIMARY KEY AUTOINCREMENT,
    "title" varchar(256) NOT NULL, "price" real NOT NULL,
    "author_id" integer NOT NULL REFERENCES "blog_author" ("id")
);

ManyToManyField: 多对多关系

使用ManyToManyField来描述多对一关系, 例如一本书有多个读者, 每个读者订阅了多本书.

class Reader(models.Model):
    name = models.CharField(max_length=256)
    books = models.ManyToManyField('Book')

我们看到多对多关系, 通过辅助表blog_reader_books来描述.

--
-- Create model Reader
--
CREATE TABLE "blog_reader" (
    "id" integer NOT NULL PRIMARY KEY AUTOINCREMENT, 
    "name" varchar(256) NOT NULL
);

CREATE TABLE "blog_reader_books" (
"id" integer NOT NULL PRIMARY KEY AUTOINCREMENT, 
"reader_id" integer NOT NULL REFERENCES "blog_reader" ("id"), 
"book_id" integer NOT NULL REFERENCES "blog_book" ("id")
);

数据访问

1. 创建 create

from blog.models import Blog
blog = Blog(title='How to learn Django', body='xxx')
blog.save()

1. 更改 update

from django.utils import timezone

blog.title = '怎样学习django'
blog.timestamp = timezone.now()
blog.hits = 30
blog.save()

1. 删除 delete

blog.delete()

1. 查询 query

# 查询全部
blog.objects.all()

# 查找某范围
blog.objects.all()[100:200]
blog.objects.all()[:100]

# 查询第一条
book = Book.objects.first() 

# 查询最后一条
book = Book.objects.last()

# get()
book = Book.objects.get(id=1)

# filter()
# 查找发布日期为2015-5-23的书.
books = Book.objects.filter(pubDate='2015-5-23')

# 查找点击量超过1000的blog
books = Blog.objects.filter(hits__gt=1000)

# 查找年龄的40-50的作者.
authors = Author.objects.filter(age__range=(40, 50))

# 查找标题中包含'how to'字样的blog
blogs = Blog.objects.filter(title__contains='how to')

# 查找作者名为'刘硕'的书
books = Book.objects.filter(author__name='刘硕')

# order_by() 以某字段排序
authors = Authors.objects.order_by('-age')
blogs = Blog.objects.filter(hits__gt='2015-5-23').order_by('hits')

# count() 获取数量
books = Blog.objects.filter(hits__gt=1000).count()

# only() defer() 指定取/不取字段
Blog.objects.only('id', 'title')
Blog.objects.defer('body')