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models

如何理解models

A model is the single, definitive source of information about your data.

It contains the essential fields and behaviors of the data you’re storing.

Generally, each model maps to a single database table.

The basics:

• Each model is a Python class that subclasses django.db.models.Model.
• Each attribute of the model represents a database field.
• With all of this, Django gives you an automatically-generated database-access API; see Making queries.

quick example

from django.db import models

class Person(models.Model):
    first_name = models.CharField(max_length=30)
    last_name = models.CharField(max_length=30)

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The above Person model would create a database table like this:

CREATE TABLE myapp_person (
    "id" serial NOT NULL PRIMARY KEY,
    "first_name" varchar(30) NOT NULL,
    "last_name" varchar(30) NOT NULL
);

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注意：表名是    ---应用名字_class名字 ，你可以重写他们

ID字段是自动添加的，并设置为主键，当然你也可以重写他们

要想使写的model在数据库生效，你需要在 INSTALLED_APPS中添加进你的应用，像这样

INSTALLED_APPS = [
    ‘django.contrib.admin‘,
    ‘django.contrib.auth‘,
    ‘django.contrib.contenttypes‘,
    ‘django.contrib.sessions‘,
    ‘django.contrib.messages‘,
    ‘django.contrib.staticfiles‘,
    ‘books.apps.BooksConfig‘,
]

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books 是我的app name, BooksConfig 是apps.py 文件中定义的类名

使用以下两条命令

python manage.py makemigrations

python manage.py migrate

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执行第一条命令后你会在migrations文件夹中看到0001_initial.py文件

manage.py migrate  这条命令是迁移到数据库，数据库中会有你创建的表

Fileds(字段)

数据表中的字段被class attributes  所指定。需要注意的是，字段的名字不要和model API 冲突，like   clear delete save

example

from django.db import models

class Musician(models.Model):
    first_name = models.CharField(max_length=50)   
    last_name = models.CharField(max_length=50)
    instrument = models.CharField(max_length=100)

class Album(models.Model):
    artist = models.ForeignKey(Musician, on_delete=models.CASCADE)
    name = models.CharField(max_length=100)
    release_date = models.DateField()  
    num_stars = models.IntegerField()

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field options

以下是字段常用的参数

• null    如果是True,  django会将空值存储为null在数据库中，默认是False
• default   The default value for the field. This can be a value or a callable object. If callable it will be called every time a new object is created.
• choice   An iterable (e.g., a list or tuple) of 2-tuples to use as choices for this field. If this is given,the default form widget will be a select box instead of the standard text field and will limit choices to the choices given.
• blank(空白)     If True, the field is allowed to be blank. Default is False.
• primary key  如果是True  ,指定这个字段是主键
• unique    如果是True    这个字段数据唯一的，不能重复

关于field name 的约束

django 对于field name 只有两个约束

1. 不能是Python的保留字，比如pass  for try 等，否则会出现语法错误
2. 字段名字不能包含超过1个下划线 (_)  这是由于django 的查询语法的工作模式导致的

class Example(models.Model):
    foo__bar = models.IntegerField() 
# ‘foo__bar‘ has two underscores!

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making query   查询

一旦你创建了数据模型，django会自动提供数据库抽象API让你创建，更新，删除object

refer to the following models

from django.db import models

class Blog(models.Model):
    name = models.CharField(max_length=100)
    tagline = models.TextField()

    def __str__(self):              # __unicode__ on Python 2
        return self.name

class Author(models.Model):
    name = models.CharField(max_length=200)
    email = models.EmailField()

    def __str__(self):              # __unicode__ on Python 2
        return self.name

class Entry(models.Model):
    blog = models.ForeignKey(Blog)
    headline = models.CharField(max_length=255)
    body_text = models.TextField()
    pub_date = models.DateField()
    mod_date = models.DateField()
    authors = models.ManyToManyField(Author)
    n_comments = models.IntegerField()
    n_pingbacks = models.IntegerField()
    rating = models.IntegerField()

    def __str__(self):              # __unicode__ on Python 2
        return self.headline

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creating object

Django uses an intuitive system:

A model class represents a database table, and an instance of that class represents a particular record in the database table.

模型类代表数据表，这个l类的实例代表这个数据表中一个特定的实例。

>>> from blog.models import Blog
>>> b = Blog(name=‘Beatles Blog‘, tagline=‘All the latest Beatles news.‘) name是blog类中的属性，即指定的字段
>>> b.save()

这个行为相当于在幕后执行了INSERT SQL 语句

这是一种插入数据的方式，还有另一种插入数据的方式：

>> Blog.object.create(name="libai",tagline="all the latest beatles news")

这种方式不需要save

Saving changes to objects

使用.save()来保存改变的对象

例如

b.name="李白"

b.save()

retrieving object(检索对象)

To retrieve objects from your database, construct a QuerySet via a Manager on your model class.

为了检索从数据库的对象，通过model类的manage方法构建一个QuerySet

关于manage方法，如果不设置名字，默认名字是objects

A QuerySet  represents a collection of objects from your database. It can have zero, one or many filters.

QuertSet 代表数据库中对象的集合，他有0个，1个或者多个过滤器  从SQL角度来看，QuerySet相当于select语句，filter相当于where  having

You get a QuerySet by using your model’s Manager. Each model has at least one Manager, and it’s called objects by default. Access it directly via the model class, like so:

NOtes：

实例是没有manage方法的

like:

>>> Blog.objects
<django.db.models.manager.Manager object at ...>
>>> b = Blog(name=‘Foo‘, tagline=‘Bar‘)
>>> b.objects
Traceback:
    ...
AttributeError: "Manager isn‘t accessible via Blog instances."

检索所有的objects

all_entries=Entry.objects.all()

很多f情况下我们需要匹配特定的，这时需要用到过滤器

Retrieving specific objects with filters

两种方法:

filter(**kwargs)

Returns a new QuerySet containing objects that match the given lookup parameters.

excludet(**kwargs)

Returns a new QuerySet containing objects that do not match the given lookup parameters.

For example, to get a QuerySet of blog entries from the year 2006, use filter() like so:

Entry.objects.filter(pub_date__year=2006)
With the default manager class, it is the same as:

Entry.objects.all().filter(pub_date__year=2006)

Retrieving a single object with get()

filter() will always give you a QuerySet, even if only a single object matches the query - in this case, it will be a QuerySetcontaining a single element.

使用get()会返回一个object

你就可以直接使用它

Django templates and models

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原文地址：http://www.cnblogs.com/yuyang26/p/7419342.html