Django学习小记[2] —

开始学习django的model了，学习django的目的很简单，就是我想用django搭建一个自己的博客，现在开源的已经有django-zinnia这个博客引擎了，但是想要看懂它，并且修改它，就必须过django这一关。之前对django的了解，仅仅限于用到了什么，就知道什么，缺乏系统的学习，所以要把django的文档都过一遍，做一下简单的笔记。

今天的主题是Model，Model就是MVC中的M，代表的数据对象，反映到数据库中就是数据表，Model中的属性是表的一列，Django对Model进行了封装，对Model提供了丰富的查询接口，反映到数据库中，就是提供了丰富的select查询功能，这就是Django的强大之处。

先来看一个简单的例子，在一个app中的models.py中，定义一个Model：

<code class="hljs python" style="font-family:Menlo,Monaco,Consolas,'Courier New',monospace;font-size:undefined; padding:0.5em; color:rgb(248,248,242); background-color:rgb(35,36,31); display:block; margin-top:0px!important"><span class="hljs-keyword" style="color:#f92672;margin-top:0px!important">from</span> django.db <span class="hljs-keyword" style="color:#f92672;">import</span> models<span class="hljs-class" style=""><span class="hljs-keyword" style="color:#66d9ef;margin-top:0px!important">class</span> <span class="hljs-title" style="">Person</span><span class="hljs-params" style="">(models.Model)</span>:</span>first_name = models.CharField(max_length=<span class="hljs-number" style="color:#ae81ff;">30</span>)last_name = models.CharField(max_length=<span class="hljs-number" style="color:#ae81ff;">30</span>)
</code>

所有的Model都继承自django.db.models.Model类，Model类的每一个属性都继承自django.db.models.Field，这个Field有几个作用：

决定该Field在数据库中的类型
决定该Field在前端上如何显示
做简单的验证

Django有很多内置的Field，当然也可以自定义。

好，下面我们来重点说一下在Django中如何实现关系型数据库的那三种典型关系：多对一，多对多，一对一

多对一

实现多对一，是使用django.db.models.ForeignKey类，ForeignKey需要一个positional的参数来指定本Model关联的Model，ForeignKey关联的Model是“一”，ForeignKey所在的Model是“多”，比如汽车和制造商的例子，一个汽车只能属于一个制造商，但是一个制造商有多个汽车，这个关系，用Django的Model来表示，就是：

<code class="hljs python" style="font-family:Menlo,Monaco,Consolas,'Courier New',monospace;font-size:undefined; padding:0.5em; color:rgb(248,248,242); background-color:rgb(35,36,31); display:block; margin-top:0px!important"><span class="hljs-class" style="margin-top:0px!important"><span class="hljs-keyword" style="color:#66d9ef;margin-top:0px!important">class</span> <span class="hljs-title" style="">Manufacturer</span><span class="hljs-params" style="">(models.Model)</span>:</span>name = models.CharField(max_length=<span class="hljs-number" style="color:#ae81ff;">30</span>)<span class="hljs-class" style=""><span class="hljs-keyword" style="color:#66d9ef;margin-top:0px!important">class</span> <span class="hljs-title" style="">Car</span><span class="hljs-params" style="">(models.Model)</span>:</span>Manufacturer = models.ForeignKey(Manufacturer)name = models.CharField(max_length=<span class="hljs-number" style="color:#ae81ff;">30</span>)
</code>

该关系，用sql语句来表示，就是：

<code class="hljs sql" style="font-family:Menlo,Monaco,Consolas,'Courier New',monospace;font-size:undefined; padding:0.5em; color:rgb(248,248,242); background-color:rgb(35,36,31); display:block; margin-top:0px!important"><span class="hljs-operator" style="margin-top:0px!important"><span class="hljs-keyword" style="color:#f92672;margin-top:0px!important">CREATE</span> <span class="hljs-keyword" style="color:#f92672;">TABLE</span> <span class="hljs-string" style="color:#e6db74;">`model_test_manufacturer`</span> (<span class="hljs-string" style="color:#e6db74;">`id`</span> <span class="hljs-keyword" style="color:#f92672;">integer</span> AUTO_INCREMENT <span class="hljs-keyword" style="color:#f92672;">NOT</span> <span class="hljs-keyword" style="color:#f92672;">NULL</span> <span class="hljs-keyword" style="color:#f92672;">PRIMARY</span> <span class="hljs-keyword" style="color:#f92672;">KEY</span>,<span class="hljs-string" style="color:#e6db74;">`name`</span> <span class="hljs-keyword" style="color:#f92672;">varchar</span>(<span class="hljs-number" style="color:#ae81ff;">30</span>) <span class="hljs-keyword" style="color:#f92672;">NOT</span> <span class="hljs-keyword" style="color:#f92672;">NULL</span>
);</span>
<span class="hljs-operator" style=""><span class="hljs-keyword" style="color:#f92672;margin-top:0px!important">CREATE</span> <span class="hljs-keyword" style="color:#f92672;">TABLE</span> <span class="hljs-string" style="color:#e6db74;">`model_test_car`</span> (<span class="hljs-string" style="color:#e6db74;">`id`</span> <span class="hljs-keyword" style="color:#f92672;">integer</span> AUTO_INCREMENT <span class="hljs-keyword" style="color:#f92672;">NOT</span> <span class="hljs-keyword" style="color:#f92672;">NULL</span> <span class="hljs-keyword" style="color:#f92672;">PRIMARY</span> <span class="hljs-keyword" style="color:#f92672;">KEY</span>,<span class="hljs-string" style="color:#e6db74;">`Manufacturer_id`</span> <span class="hljs-keyword" style="color:#f92672;">integer</span> <span class="hljs-keyword" style="color:#f92672;">NOT</span> <span class="hljs-keyword" style="color:#f92672;">NULL</span>,<span class="hljs-string" style="color:#e6db74;">`name`</span> <span class="hljs-keyword" style="color:#f92672;">varchar</span>(<span class="hljs-number" style="color:#ae81ff;">30</span>) <span class="hljs-keyword" style="color:#f92672;">NOT</span> <span class="hljs-keyword" style="color:#f92672;">NULL</span>
);</span>
<span class="hljs-operator" style=""><span class="hljs-keyword" style="color:#f92672;margin-top:0px!important">ALTER</span> <span class="hljs-keyword" style="color:#f92672;">TABLE</span> <span class="hljs-string" style="color:#e6db74;">`model_test_car`</span> <span class="hljs-keyword" style="color:#f92672;">ADD</span> <span class="hljs-keyword" style="color:#f92672;">CONSTRAINT</span> <span class="hljs-string" style="color:#e6db74;">`Manufacturer_id_refs_id_da7168cb`</span> <span class="hljs-keyword" style="color:#f92672;">FOREIGN</span> <span class="hljs-keyword" style="color:#f92672;">KEY</span> (<span class="hljs-string" style="color:#e6db74;">`Manufacturer_id`</span>) <span class="hljs-keyword" style="color:#f92672;">REFERENCES</span> <span class="hljs-string" style="color:#e6db74;">`model_test_manufacturer`</span> (<span class="hljs-string" style="color:#e6db74;">`id`</span>);</span>
</code>

增删操作：

<code class="hljs python" style="font-family:Menlo,Monaco,Consolas,'Courier New',monospace;font-size:undefined; padding:0.5em; color:rgb(248,248,242); background-color:rgb(35,36,31); display:block; margin-top:0px!important"><span class="hljs-prompt" style="color:#e6db74;margin-top:0px!important">>>> </span><span class="hljs-keyword" style="color:#f92672;">from</span> model_test.models <span class="hljs-keyword" style="color:#f92672;">import</span> Car
<span class="hljs-prompt" style="color:#e6db74;">>>> </span><span class="hljs-keyword" style="color:#f92672;">from</span> model_test.models <span class="hljs-keyword" style="color:#f92672;">import</span> Manufacturer
<span class="hljs-prompt" style="color:#e6db74;">>>> </span>m = Manufacturer.objects.get(name=<span class="hljs-string" style="color:#e6db74;">"xxx"</span>)
<span class="hljs-prompt" style="color:#e6db74;">>>> </span>m.car_set.all()
[]
<span class="hljs-prompt" style="color:#e6db74;">>>> </span>m.car_set.create(name=<span class="hljs-string" style="color:#e6db74;">"yyy"</span>)
<Car: Car object>
<span class="hljs-prompt" style="color:#e6db74;">>>> </span>c = Car(name=<span class="hljs-string" style="color:#e6db74;">"zzz"</span>)
<span class="hljs-prompt" style="color:#e6db74;">>>> </span>m.car_set.add(c)
</code>

关于多对一更多的内容，参考：Many-to-One

多对多

要实现多对多，就要使用django.db.models.ManyToManyField类，和ForeignKey一样，它也有一个positional的参数，用来指定和它关联的Model。

如果不仅仅需要知道两个Model之间是多对多的关系，还需要知道这个关系的更多信息，比如Person和Group是多对多的关系，除了知道一个Person属于哪个Group之外，如果还想知道这个Person是什么时候加入这个Group的，那么就需要有一个中间表来记录这些信息，那就用到了ManyToManyFiled的一个optional参数: through，如下面的例子:

<code class="hljs python" style="font-family:Menlo,Monaco,Consolas,'Courier New',monospace;font-size:undefined; padding:0.5em; color:rgb(248,248,242); background-color:rgb(35,36,31); display:block; margin-top:0px!important"><span class="hljs-class" style="margin-top:0px!important"><span class="hljs-keyword" style="color:#66d9ef;margin-top:0px!important">class</span> <span class="hljs-title" style="">Person</span><span class="hljs-params" style="">(models.Model)</span>:</span>                        name = models.CharField(max_length=<span class="hljs-number" style="color:#ae81ff;">30</span>)<span class="hljs-function" style="color:#f92672;"><span class="hljs-keyword" style="color:#66d9ef;margin-top:0px!important">def</span> <span class="hljs-title" style="color:#a6e22e;">__unicode__</span><span class="hljs-params" style="color:#f8f8f2;">(self)</span>:</span>                         <span class="hljs-keyword" style="color:#f92672;">return</span> self.name<span class="hljs-class" style=""><span class="hljs-keyword" style="color:#66d9ef;margin-top:0px!important">class</span> <span class="hljs-title" style="">Group</span><span class="hljs-params" style="">(models.Model)</span>:</span>name = models.CharField(max_length=<span class="hljs-number" style="color:#ae81ff;">30</span>)members = models.ManyToManyField(Person, through=<span class="hljs-string" style="color:#e6db74;">'Membership'</span>)<span class="hljs-function" style="color:#f92672;"><span class="hljs-keyword" style="color:#66d9ef;margin-top:0px!important">def</span> <span class="hljs-title" style="color:#a6e22e;">__unicode__</span><span class="hljs-params" style="color:#f8f8f2;">(self)</span>:</span><span class="hljs-keyword" style="color:#f92672;">return</span> self.name<span class="hljs-class" style=""><span class="hljs-keyword" style="color:#66d9ef;margin-top:0px!important">class</span> <span class="hljs-title" style="">Membership</span><span class="hljs-params" style="">(models.Model)</span>:</span>person = models.ForeignKey(Person)group = models.ForeignKey(Group)date_joined = models.DateField()invite_person = models.CharField(max_length=<span class="hljs-number" style="color:#ae81ff;">30</span>)
</code>

在中间表中，通过外键关联到Person和Group，其实，就是两个多对一的关系，上面对应的SQL语句为:

<code class="hljs sql" style="font-family:Menlo,Monaco,Consolas,'Courier New',monospace;font-size:undefined; padding:0.5em; color:rgb(248,248,242); background-color:rgb(35,36,31); display:block; margin-top:0px!important"><span class="hljs-operator" style="margin-top:0px!important"><span class="hljs-keyword" style="color:#f92672;margin-top:0px!important">CREATE</span> <span class="hljs-keyword" style="color:#f92672;">TABLE</span> <span class="hljs-string" style="color:#e6db74;">`model_test_person`</span> (<span class="hljs-string" style="color:#e6db74;">`id`</span> <span class="hljs-keyword" style="color:#f92672;">integer</span> AUTO_INCREMENT <span class="hljs-keyword" style="color:#f92672;">NOT</span> <span class="hljs-keyword" style="color:#f92672;">NULL</span> <span class="hljs-keyword" style="color:#f92672;">PRIMARY</span> <span class="hljs-keyword" style="color:#f92672;">KEY</span>,<span class="hljs-string" style="color:#e6db74;">`name`</span> <span class="hljs-keyword" style="color:#f92672;">varchar</span>(<span class="hljs-number" style="color:#ae81ff;">30</span>) <span class="hljs-keyword" style="color:#f92672;">NOT</span> <span class="hljs-keyword" style="color:#f92672;">NULL</span>
)
;</span>
<span class="hljs-operator" style=""><span class="hljs-keyword" style="color:#f92672;margin-top:0px!important">CREATE</span> <span class="hljs-keyword" style="color:#f92672;">TABLE</span> <span class="hljs-string" style="color:#e6db74;">`model_test_group`</span> (<span class="hljs-string" style="color:#e6db74;">`id`</span> <span class="hljs-keyword" style="color:#f92672;">integer</span> AUTO_INCREMENT <span class="hljs-keyword" style="color:#f92672;">NOT</span> <span class="hljs-keyword" style="color:#f92672;">NULL</span> <span class="hljs-keyword" style="color:#f92672;">PRIMARY</span> <span class="hljs-keyword" style="color:#f92672;">KEY</span>,<span class="hljs-string" style="color:#e6db74;">`name`</span> <span class="hljs-keyword" style="color:#f92672;">varchar</span>(<span class="hljs-number" style="color:#ae81ff;">30</span>) <span class="hljs-keyword" style="color:#f92672;">NOT</span> <span class="hljs-keyword" style="color:#f92672;">NULL</span>
)
;</span>
<span class="hljs-operator" style=""><span class="hljs-keyword" style="color:#f92672;margin-top:0px!important">CREATE</span> <span class="hljs-keyword" style="color:#f92672;">TABLE</span> <span class="hljs-string" style="color:#e6db74;">`model_test_membership`</span> (<span class="hljs-string" style="color:#e6db74;">`id`</span> <span class="hljs-keyword" style="color:#f92672;">integer</span> AUTO_INCREMENT <span class="hljs-keyword" style="color:#f92672;">NOT</span> <span class="hljs-keyword" style="color:#f92672;">NULL</span> <span class="hljs-keyword" style="color:#f92672;">PRIMARY</span> <span class="hljs-keyword" style="color:#f92672;">KEY</span>,<span class="hljs-string" style="color:#e6db74;">`person_id`</span> <span class="hljs-keyword" style="color:#f92672;">integer</span> <span class="hljs-keyword" style="color:#f92672;">NOT</span> <span class="hljs-keyword" style="color:#f92672;">NULL</span>,<span class="hljs-string" style="color:#e6db74;">`group_id`</span> <span class="hljs-keyword" style="color:#f92672;">integer</span> <span class="hljs-keyword" style="color:#f92672;">NOT</span> <span class="hljs-keyword" style="color:#f92672;">NULL</span>,<span class="hljs-string" style="color:#e6db74;">`date_joined`</span> <span class="hljs-keyword" style="color:#f92672;">date</span> <span class="hljs-keyword" style="color:#f92672;">NOT</span> <span class="hljs-keyword" style="color:#f92672;">NULL</span>,<span class="hljs-string" style="color:#e6db74;">`invite_person`</span> <span class="hljs-keyword" style="color:#f92672;">varchar</span>(<span class="hljs-number" style="color:#ae81ff;">30</span>) <span class="hljs-keyword" style="color:#f92672;">NOT</span> <span class="hljs-keyword" style="color:#f92672;">NULL</span>
)
;</span>
<span class="hljs-operator" style=""><span class="hljs-keyword" style="color:#f92672;margin-top:0px!important">ALTER</span> <span class="hljs-keyword" style="color:#f92672;">TABLE</span> <span class="hljs-string" style="color:#e6db74;">`model_test_membership`</span> <span class="hljs-keyword" style="color:#f92672;">ADD</span> <span class="hljs-keyword" style="color:#f92672;">CONSTRAINT</span> <span class="hljs-string" style="color:#e6db74;">`group_id_refs_id_be33a6a7`</span> <span class="hljs-keyword" style="color:#f92672;">FOREIGN</span> <span class="hljs-keyword" style="color:#f92672;">KEY</span> (<span class="hljs-string" style="color:#e6db74;">`group_id`</span>) <span class="hljs-keyword" style="color:#f92672;">REFERENCES</span> <span class="hljs-string" style="color:#e6db74;">`model_test_group`</span> (<span class="hljs-string" style="color:#e6db74;">`id`</span>);</span>
<span class="hljs-operator" style=""><span class="hljs-keyword" style="color:#f92672;margin-top:0px!important">ALTER</span> <span class="hljs-keyword" style="color:#f92672;">TABLE</span> <span class="hljs-string" style="color:#e6db74;">`model_test_membership`</span> <span class="hljs-keyword" style="color:#f92672;">ADD</span> <span class="hljs-keyword" style="color:#f92672;">CONSTRAINT</span> <span class="hljs-string" style="color:#e6db74;">`person_id_refs_id_90aaf3d5`</span> <span class="hljs-keyword" style="color:#f92672;">FOREIGN</span> <span class="hljs-keyword" style="color:#f92672;">KEY</span> (<span class="hljs-string" style="color:#e6db74;">`person_id`</span>) <span class="hljs-keyword" style="color:#f92672;">REFERENCES</span> <span class="hljs-string" style="color:#e6db74;">`model_test_person`</span> (<span class="hljs-string" style="color:#e6db74;">`id`</span>);</span>
</code>

对Model进行增加/删除操作：

<code class="hljs python" style="font-family:Menlo,Monaco,Consolas,'Courier New',monospace;font-size:undefined; padding:0.5em; color:rgb(248,248,242); background-color:rgb(35,36,31); display:block; margin-top:0px!important"><span class="hljs-prompt" style="color:#e6db74;margin-top:0px!important">>>> </span><span class="hljs-keyword" style="color:#f92672;">import</span> datetime
<span class="hljs-prompt" style="color:#e6db74;">>>> </span><span class="hljs-keyword" style="color:#f92672;">from</span> model_test.models <span class="hljs-keyword" style="color:#f92672;">import</span> Person
<span class="hljs-prompt" style="color:#e6db74;">>>> </span><span class="hljs-keyword" style="color:#f92672;">from</span> model_test.models <span class="hljs-keyword" style="color:#f92672;">import</span> Group
<span class="hljs-prompt" style="color:#e6db74;">>>> </span><span class="hljs-keyword" style="color:#f92672;">from</span> model_test.models <span class="hljs-keyword" style="color:#f92672;">import</span> Membership
<span class="hljs-prompt" style="color:#e6db74;">>>> </span>
<span class="hljs-prompt" style="color:#e6db74;">>>> </span>suo = Person.objects.create(name=<span class="hljs-string" style="color:#e6db74;">"suo"</span>)
<span class="hljs-prompt" style="color:#e6db74;">>>> </span>piao = Person.objects.create(name=<span class="hljs-string" style="color:#e6db74;">"piao"</span>)
<span class="hljs-prompt" style="color:#e6db74;">>>> </span>english = Group.objects.create(name=<span class="hljs-string" style="color:#e6db74;">"English"</span>)
<span class="hljs-prompt" style="color:#e6db74;">>>> </span>
<span class="hljs-prompt" style="color:#e6db74;">>>> </span>m1 = Membership(person=suo, group=english, date_joined=datetime.date(<span class="hljs-number" style="color:#ae81ff;">2014</span>, <span class="hljs-number" style="color:#ae81ff;">9</span>, <span class="hljs-number" style="color:#ae81ff;">9</span>), invite_person=<span class="hljs-string" style="color:#e6db74;">"summer"</span>)
<span class="hljs-prompt" style="color:#e6db74;">>>> </span>m1.save()
<span class="hljs-prompt" style="color:#e6db74;">>>> </span>m2 = Membership.objects.create(person=piao, group=english, date_joined=datetime.date(<span class="hljs-number" style="color:#ae81ff;">2014</span>, <span class="hljs-number" style="color:#ae81ff;">8</span>, <span class="hljs-number" style="color:#ae81ff;">8</span>), invite_person=<span class="hljs-string" style="color:#e6db74;">"spring"</span>)
<span class="hljs-prompt" style="color:#e6db74;">>>> </span>
<span class="hljs-prompt" style="color:#e6db74;">>>> </span>english.members.all()
[<Person: suo>, <Person: piao>]
</code>

注意，这种形式的多对多，添加两个Model的关系时，不能够通过Model的关联属性直接添加，而应该是创建中间关系的对象，即不能执行这样的操作：

<code style="font-family:Menlo,Monaco,Consolas,'Courier New',monospace;font-size:undefined; padding:0px; color:inherit; background-color:transparent; margin-top:0px!important">english.members.add(suo)
</code>

因为添加关系还有其他的属性(date_jointed/invite_person)需要指定，所以不能够直接添加。

一对一

一对一是通过django.db.models.OneToOneField来实现的，被关联的Model会被加上Unique的限制。比如Person和IdCard就是一对一的关系，用Django的Model来表示，就是：

<code class="hljs python" style="font-family:Menlo,Monaco,Consolas,'Courier New',monospace;font-size:undefined; padding:0.5em; color:rgb(248,248,242); background-color:rgb(35,36,31); display:block; margin-top:0px!important"><span class="hljs-class" style="margin-top:0px!important"><span class="hljs-keyword" style="color:#66d9ef;margin-top:0px!important">class</span> <span class="hljs-title" style="">IdCard</span><span class="hljs-params" style="">(models.Model)</span>:</span>number = models.CharField(max_length=<span class="hljs-number" style="color:#ae81ff;">30</span>)person = models.OneToOneField(Person)<span class="hljs-class" style=""><span class="hljs-keyword" style="color:#66d9ef;margin-top:0px!important">class</span> <span class="hljs-title" style="">Person</span><span class="hljs-params" style="">(models.Model)</span>:</span>name = models.CharField(max_length=<span class="hljs-number" style="color:#ae81ff;">30</span>)<span class="hljs-function" style="color:#f92672;"><span class="hljs-keyword" style="color:#66d9ef;margin-top:0px!important">def</span> <span class="hljs-title" style="color:#a6e22e;">__unicode__</span><span class="hljs-params" style="color:#f8f8f2;">(self)</span>:</span><span class="hljs-keyword" style="color:#f92672;">return</span> self.name
</code>

对应的SQL语句为：

<code class="hljs sql" style="font-family:Menlo,Monaco,Consolas,'Courier New',monospace;font-size:undefined; padding:0.5em; color:rgb(248,248,242); background-color:rgb(35,36,31); display:block; margin-top:0px!important"><span class="hljs-operator" style="margin-top:0px!important"><span class="hljs-keyword" style="color:#f92672;margin-top:0px!important">CREATE</span> <span class="hljs-keyword" style="color:#f92672;">TABLE</span> <span class="hljs-string" style="color:#e6db74;">`model_test_person`</span> (<span class="hljs-string" style="color:#e6db74;">`id`</span> <span class="hljs-keyword" style="color:#f92672;">integer</span> AUTO_INCREMENT <span class="hljs-keyword" style="color:#f92672;">NOT</span> <span class="hljs-keyword" style="color:#f92672;">NULL</span> <span class="hljs-keyword" style="color:#f92672;">PRIMARY</span> <span class="hljs-keyword" style="color:#f92672;">KEY</span>,<span class="hljs-string" style="color:#e6db74;">`name`</span> <span class="hljs-keyword" style="color:#f92672;">varchar</span>(<span class="hljs-number" style="color:#ae81ff;">30</span>) <span class="hljs-keyword" style="color:#f92672;">NOT</span> <span class="hljs-keyword" style="color:#f92672;">NULL</span>
)
;</span>
<span class="hljs-operator" style=""><span class="hljs-keyword" style="color:#f92672;margin-top:0px!important">CREATE</span> <span class="hljs-keyword" style="color:#f92672;">TABLE</span> <span class="hljs-string" style="color:#e6db74;">`model_test_idcard`</span> (<span class="hljs-string" style="color:#e6db74;">`id`</span> <span class="hljs-keyword" style="color:#f92672;">integer</span> AUTO_INCREMENT <span class="hljs-keyword" style="color:#f92672;">NOT</span> <span class="hljs-keyword" style="color:#f92672;">NULL</span> <span class="hljs-keyword" style="color:#f92672;">PRIMARY</span> <span class="hljs-keyword" style="color:#f92672;">KEY</span>,<span class="hljs-string" style="color:#e6db74;">`number`</span> <span class="hljs-keyword" style="color:#f92672;">varchar</span>(<span class="hljs-number" style="color:#ae81ff;">30</span>) <span class="hljs-keyword" style="color:#f92672;">NOT</span> <span class="hljs-keyword" style="color:#f92672;">NULL</span>,<span class="hljs-string" style="color:#e6db74;">`person_id`</span> <span class="hljs-keyword" style="color:#f92672;">integer</span> <span class="hljs-keyword" style="color:#f92672;">NOT</span> <span class="hljs-keyword" style="color:#f92672;">NULL</span> <span class="hljs-keyword" style="color:#f92672;">UNIQUE</span>
)
;</span>
<span class="hljs-operator" style=""><span class="hljs-keyword" style="color:#f92672;margin-top:0px!important">ALTER</span> <span class="hljs-keyword" style="color:#f92672;">TABLE</span> <span class="hljs-string" style="color:#e6db74;">`model_test_idcard`</span> <span class="hljs-keyword" style="color:#f92672;">ADD</span> <span class="hljs-keyword" style="color:#f92672;">CONSTRAINT</span> <span class="hljs-string" style="color:#e6db74;">`person_id_refs_id_c2c57084`</span> <span class="hljs-keyword" style="color:#f92672;">FOREIGN</span> <span class="hljs-keyword" style="color:#f92672;">KEY</span> (<span class="hljs-string" style="color:#e6db74;">`person_id`</span>) <span class="hljs-keyword" style="color:#f92672;">REFERENCES</span> <span class="hljs-string" style="color:#e6db74;">`model_test_person`</span> (<span class="hljs-string" style="color:#e6db74;">`id`</span>);</span>
</code>

注意model_test_idcard表的person_id加上了unique限制。

接下来，再来介绍一下Model的继承，有三种：Abstract base classes, Multi-table inheritance, Proxy models

Abstract base classes

Model之间的继承关系其实也就是Python中的继承，子类继承父类中的属性，但是由于每一个Model都对应了数据库中的一个数据表，所以有些地方得需要做一些特殊处理：做为父类的Model要在它的Meta中显示的申明为抽象，否则也会为父类创建数据库表，一般情况下，我们只需要父类做为一个存放公共代码的地方，并不想要它有自己的数据库表。需要注意的是，子类继承父类中的属性，包括Meta中的属性，但是唯独不继承Meta中的abstract属性，如果子类也想要是抽象Model，那么要显示的再次指定该参数。如下面的例子：

<code class="hljs python" style="font-family:Menlo,Monaco,Consolas,'Courier New',monospace;font-size:undefined; padding:0.5em; color:rgb(248,248,242); background-color:rgb(35,36,31); display:block; margin-top:0px!important"><span class="hljs-class" style="margin-top:0px!important"><span class="hljs-keyword" style="color:#66d9ef;margin-top:0px!important">class</span> <span class="hljs-title" style="">CommonInfo</span><span class="hljs-params" style="">(models.Model)</span>:</span>name = models.CharField(max_length=<span class="hljs-number" style="color:#ae81ff;">30</span>)age = models.PositiveIntegerField()<span class="hljs-class" style=""><span class="hljs-keyword" style="color:#66d9ef;margin-top:0px!important">class</span> <span class="hljs-title" style="color:#a6e22e;font-style:italic">Meta</span>:</span>abstract = <span class="hljs-keyword" style="color:#f92672;">True</span><span class="hljs-class" style=""><span class="hljs-keyword" style="color:#66d9ef;margin-top:0px!important">class</span> <span class="hljs-title" style="">Student</span><span class="hljs-params" style="">(CommonInfo)</span>:</span>score = models.IntegerField()<span class="hljs-class" style=""><span class="hljs-keyword" style="color:#66d9ef;margin-top:0px!important">class</span> <span class="hljs-title" style="">Teacher</span><span class="hljs-params" style="">(CommonInfo)</span>:</span>rating = models.CharField(max_length=<span class="hljs-number" style="color:#ae81ff;">30</span>)
</code>

对应的SQL语句为：

<code class="hljs sql" style="font-family:Menlo,Monaco,Consolas,'Courier New',monospace;font-size:undefined; padding:0.5em; color:rgb(248,248,242); background-color:rgb(35,36,31); display:block; margin-top:0px!important"><span class="hljs-operator" style="margin-top:0px!important"><span class="hljs-keyword" style="color:#f92672;margin-top:0px!important">CREATE</span> <span class="hljs-keyword" style="color:#f92672;">TABLE</span> <span class="hljs-string" style="color:#e6db74;">`model_test_student`</span> (<span class="hljs-string" style="color:#e6db74;">`id`</span> <span class="hljs-keyword" style="color:#f92672;">integer</span> AUTO_INCREMENT <span class="hljs-keyword" style="color:#f92672;">NOT</span> <span class="hljs-keyword" style="color:#f92672;">NULL</span> <span class="hljs-keyword" style="color:#f92672;">PRIMARY</span> <span class="hljs-keyword" style="color:#f92672;">KEY</span>,<span class="hljs-string" style="color:#e6db74;">`name`</span> <span class="hljs-keyword" style="color:#f92672;">varchar</span>(<span class="hljs-number" style="color:#ae81ff;">30</span>) <span class="hljs-keyword" style="color:#f92672;">NOT</span> <span class="hljs-keyword" style="color:#f92672;">NULL</span>,<span class="hljs-string" style="color:#e6db74;">`age`</span> <span class="hljs-keyword" style="color:#f92672;">integer</span> UNSIGNED <span class="hljs-keyword" style="color:#f92672;">NOT</span> <span class="hljs-keyword" style="color:#f92672;">NULL</span>,<span class="hljs-string" style="color:#e6db74;">`score`</span> <span class="hljs-keyword" style="color:#f92672;">integer</span> <span class="hljs-keyword" style="color:#f92672;">NOT</span> <span class="hljs-keyword" style="color:#f92672;">NULL</span>
)
;</span>
<span class="hljs-operator" style=""><span class="hljs-keyword" style="color:#f92672;margin-top:0px!important">CREATE</span> <span class="hljs-keyword" style="color:#f92672;">TABLE</span> <span class="hljs-string" style="color:#e6db74;">`model_test_teacher`</span> (<span class="hljs-string" style="color:#e6db74;">`id`</span> <span class="hljs-keyword" style="color:#f92672;">integer</span> AUTO_INCREMENT <span class="hljs-keyword" style="color:#f92672;">NOT</span> <span class="hljs-keyword" style="color:#f92672;">NULL</span> <span class="hljs-keyword" style="color:#f92672;">PRIMARY</span> <span class="hljs-keyword" style="color:#f92672;">KEY</span>,<span class="hljs-string" style="color:#e6db74;">`name`</span> <span class="hljs-keyword" style="color:#f92672;">varchar</span>(<span class="hljs-number" style="color:#ae81ff;">30</span>) <span class="hljs-keyword" style="color:#f92672;">NOT</span> <span class="hljs-keyword" style="color:#f92672;">NULL</span>,<span class="hljs-string" style="color:#e6db74;">`age`</span> <span class="hljs-keyword" style="color:#f92672;">integer</span> UNSIGNED <span class="hljs-keyword" style="color:#f92672;">NOT</span> <span class="hljs-keyword" style="color:#f92672;">NULL</span>,<span class="hljs-string" style="color:#e6db74;">`rating`</span> <span class="hljs-keyword" style="color:#f92672;">varchar</span>(<span class="hljs-number" style="color:#ae81ff;">30</span>) <span class="hljs-keyword" style="color:#f92672;">NOT</span> <span class="hljs-keyword" style="color:#f92672;">NULL</span>
)
;</span>
</code>

Multi-table inheritance

这种和上面的区别就是把父类中的Meta的abstract去掉了，也就是说父类也有自己的数据库表，而且这个父类和子类之间是一对一的关系。例子如下：

<code class="hljs python" style="font-family:Menlo,Monaco,Consolas,'Courier New',monospace;font-size:undefined; padding:0.5em; color:rgb(248,248,242); background-color:rgb(35,36,31); display:block; margin-top:0px!important"><span class="hljs-class" style="margin-top:0px!important"><span class="hljs-keyword" style="color:#66d9ef;margin-top:0px!important">class</span> <span class="hljs-title" style="">Place</span><span class="hljs-params" style="">(models.Model)</span>:</span>name = models.CharField(max_length=<span class="hljs-number" style="color:#ae81ff;">30</span>)address = models.CharField(max_length=<span class="hljs-number" style="color:#ae81ff;">30</span>)<span class="hljs-function" style="color:#f92672;"><span class="hljs-keyword" style="color:#66d9ef;margin-top:0px!important">def</span> <span class="hljs-title" style="color:#a6e22e;">__unicode__</span><span class="hljs-params" style="color:#f8f8f2;">(self)</span>:</span><span class="hljs-keyword" style="color:#f92672;">return</span> self.name<span class="hljs-class" style=""><span class="hljs-keyword" style="color:#66d9ef;margin-top:0px!important">class</span> <span class="hljs-title" style="">Restaurant</span><span class="hljs-params" style="">(Place)</span>:</span>serves_hot_dogs = models.BooleanField()serves_pizza = models.BooleanField()
</code>

得到的SQL如下：

<code class="hljs sql" style="font-family:Menlo,Monaco,Consolas,'Courier New',monospace;font-size:undefined; padding:0.5em; color:rgb(248,248,242); background-color:rgb(35,36,31); display:block; margin-top:0px!important"><span class="hljs-operator" style="margin-top:0px!important"><span class="hljs-keyword" style="color:#f92672;margin-top:0px!important">CREATE</span> <span class="hljs-keyword" style="color:#f92672;">TABLE</span> <span class="hljs-string" style="color:#e6db74;">`model_test_place`</span> (<span class="hljs-string" style="color:#e6db74;">`id`</span> <span class="hljs-keyword" style="color:#f92672;">integer</span> AUTO_INCREMENT <span class="hljs-keyword" style="color:#f92672;">NOT</span> <span class="hljs-keyword" style="color:#f92672;">NULL</span> <span class="hljs-keyword" style="color:#f92672;">PRIMARY</span> <span class="hljs-keyword" style="color:#f92672;">KEY</span>,<span class="hljs-string" style="color:#e6db74;">`name`</span> <span class="hljs-keyword" style="color:#f92672;">varchar</span>(<span class="hljs-number" style="color:#ae81ff;">30</span>) <span class="hljs-keyword" style="color:#f92672;">NOT</span> <span class="hljs-keyword" style="color:#f92672;">NULL</span>,<span class="hljs-string" style="color:#e6db74;">`address`</span> <span class="hljs-keyword" style="color:#f92672;">varchar</span>(<span class="hljs-number" style="color:#ae81ff;">30</span>) <span class="hljs-keyword" style="color:#f92672;">NOT</span> <span class="hljs-keyword" style="color:#f92672;">NULL</span>
)
;</span>
<span class="hljs-operator" style=""><span class="hljs-keyword" style="color:#f92672;margin-top:0px!important">CREATE</span> <span class="hljs-keyword" style="color:#f92672;">TABLE</span> <span class="hljs-string" style="color:#e6db74;">`model_test_restaurant`</span> (<span class="hljs-string" style="color:#e6db74;">`place_ptr_id`</span> <span class="hljs-keyword" style="color:#f92672;">integer</span> <span class="hljs-keyword" style="color:#f92672;">NOT</span> <span class="hljs-keyword" style="color:#f92672;">NULL</span> <span class="hljs-keyword" style="color:#f92672;">PRIMARY</span> <span class="hljs-keyword" style="color:#f92672;">KEY</span>,<span class="hljs-string" style="color:#e6db74;">`serves_hot_dogs`</span> bool <span class="hljs-keyword" style="color:#f92672;">NOT</span> <span class="hljs-keyword" style="color:#f92672;">NULL</span>,<span class="hljs-string" style="color:#e6db74;">`serves_pizza`</span> bool <span class="hljs-keyword" style="color:#f92672;">NOT</span> <span class="hljs-keyword" style="color:#f92672;">NULL</span>
)
;</span>
<span class="hljs-operator" style=""><span class="hljs-keyword" style="color:#f92672;margin-top:0px!important">ALTER</span> <span class="hljs-keyword" style="color:#f92672;">TABLE</span> <span class="hljs-string" style="color:#e6db74;">`model_test_restaurant`</span> <span class="hljs-keyword" style="color:#f92672;">ADD</span> <span class="hljs-keyword" style="color:#f92672;">CONSTRAINT</span> <span class="hljs-string" style="color:#e6db74;">`place_ptr_id_refs_id_cc7b5838`</span> <span class="hljs-keyword" style="color:#f92672;">FOREIGN</span> <span class="hljs-keyword" style="color:#f92672;">KEY</span> (<span class="hljs-string" style="color:#e6db74;">`place_ptr_id`</span>) <span class="hljs-keyword" style="color:#f92672;">REFERENCES</span> <span class="hljs-string" style="color:#e6db74;">`model_test_place`</span> (<span class="hljs-string" style="color:#e6db74;">`id`</span>);</span>
</code>

添加一个Place很容易，那么怎么来添加一个Restaurant呢？而且两者是一对一的关系，怎么来添加他们之间的关系呢？记住，Restaurant是Place的子类，继承了Place的属性，所以直接创建Restaurant就可以了：

<code class="hljs python" style="font-family:Menlo,Monaco,Consolas,'Courier New',monospace;font-size:undefined; padding:0.5em; color:rgb(248,248,242); background-color:rgb(35,36,31); display:block; margin-top:0px!important"><span class="hljs-prompt" style="color:#e6db74;margin-top:0px!important">>>> </span><span class="hljs-keyword" style="color:#f92672;">from</span> model_test.models <span class="hljs-keyword" style="color:#f92672;">import</span> Restaurant
<span class="hljs-prompt" style="color:#e6db74;">>>> </span>Restaurant.objects.create(name=<span class="hljs-string" style="color:#e6db74;">"r1"</span>, address=<span class="hljs-string" style="color:#e6db74;">"a1"</span>, serves_hot_dogs=<span class="hljs-keyword" style="color:#f92672;">True</span>, serves_pizza=<span class="hljs-keyword" style="color:#f92672;">False</span>)
<Restaurant: r1>
</code>

这样，在数据库中，就会分别向place和restaurant表中各添加一条记录，而且restaurant表中用place id作为主键。

更多关于这个类型的内容见：Multi-table inheritance

Proxy models

这种类型的继承用的比较少，它主要用来在不改变原来Model的行为的情况下，扩展Model的行为，即为原来的Model设置了一个代理，可以重新定义该代理的行为，但是保留原来Model的行为。比如说原来的Model我想让它是一种排序方法，但是我也想让它有另外一种排序方法怎么办？那就为该Model创建一个代理，在这个代理中指定另外一个排序方法，通过这个代理来访问，就可以得到新的排序。

这里有个限制，就是父Model不能是抽象的。举个例子：

<code class="hljs python" style="font-family:Menlo,Monaco,Consolas,'Courier New',monospace;font-size:undefined; padding:0.5em; color:rgb(248,248,242); background-color:rgb(35,36,31); display:block; margin-top:0px!important"><span class="hljs-class" style="margin-top:0px!important"><span class="hljs-keyword" style="color:#66d9ef;margin-top:0px!important">class</span> <span class="hljs-title" style="">Person</span><span class="hljs-params" style="">(models.Model)</span>:</span>first_name = models.CharField(max_length=<span class="hljs-number" style="color:#ae81ff;">30</span>)last_name = models.CharField(max_length=<span class="hljs-number" style="color:#ae81ff;">30</span>)<span class="hljs-class" style=""><span class="hljs-keyword" style="color:#66d9ef;margin-top:0px!important">class</span> <span class="hljs-title" style="color:#a6e22e;font-style:italic">Meta</span>:</span>ordering = [<span class="hljs-string" style="color:#e6db74;">'first_name'</span>]<span class="hljs-function" style="color:#f92672;"><span class="hljs-keyword" style="color:#66d9ef;margin-top:0px!important">def</span> <span class="hljs-title" style="color:#a6e22e;">__unicode__</span><span class="hljs-params" style="color:#f8f8f2;">(self)</span>:</span><span class="hljs-keyword" style="color:#f92672;">return</span> self.name<span class="hljs-class" style=""><span class="hljs-keyword" style="color:#66d9ef;margin-top:0px!important">class</span> <span class="hljs-title" style="">MyPerson</span><span class="hljs-params" style="">(Person)</span>:</span><span class="hljs-class" style=""><span class="hljs-keyword" style="color:#66d9ef;margin-top:0px!important">class</span> <span class="hljs-title" style="color:#a6e22e;font-style:italic">Meta</span>:</span>proxy = <span class="hljs-keyword" style="color:#f92672;">True</span>ordering = [<span class="hljs-string" style="color:#e6db74;">'last_name'</span>]<span class="hljs-function" style="color:#f92672;"><span class="hljs-keyword" style="color:#66d9ef;margin-top:0px!important">def</span> <span class="hljs-title" style="color:#a6e22e;">do_something</span><span class="hljs-params" style="color:#f8f8f2;">(self)</span>:</span><span class="hljs-keyword" style="color:#f92672;">pass</span>
</code>

在子类中的Meta中设置了proxy=True，就是指定该Model为代理Model，他们两个对应同一个数据库，但是有不同的访问行为。通过MyPerson访问的数据，会按last_name进行排序，而且还可以通过定义新的方法，扩展它的功能。如：

<code class="hljs python" style="font-family:Menlo,Monaco,Consolas,'Courier New',monospace;font-size:undefined; padding:0.5em; color:rgb(248,248,242); background-color:rgb(35,36,31); display:block; margin-top:0px!important"><span class="hljs-prompt" style="color:#e6db74;margin-top:0px!important">>>> </span><span class="hljs-keyword" style="color:#f92672;">from</span> model_test.models <span class="hljs-keyword" style="color:#f92672;">import</span> Person
<span class="hljs-prompt" style="color:#e6db74;">>>> </span><span class="hljs-keyword" style="color:#f92672;">from</span> model_test.models <span class="hljs-keyword" style="color:#f92672;">import</span> MyPerson<span class="hljs-prompt" style="color:#e6db74;">>>> </span>Person.objects.create(first_name=<span class="hljs-string" style="color:#e6db74;">"suo"</span>, last_name=<span class="hljs-string" style="color:#e6db74;">"guangyu"</span>)
<Person: suo guangyu>
<span class="hljs-prompt" style="color:#e6db74;">>>> </span>Person.objects.create(first_name=<span class="hljs-string" style="color:#e6db74;">"xing"</span>, last_name=<span class="hljs-string" style="color:#e6db74;">"demo"</span>)
<Person: xing demo><span class="hljs-prompt" style="color:#e6db74;">>>> </span>Person.objects.all()
[<Person: suo guangyu>, <Person: xing demo>]
<span class="hljs-prompt" style="color:#e6db74;">>>> </span>MyPerson.objects.all()
[<MyPerson: xing demo>, <MyPerson: suo guangyu>]
>>>
</code>

这个类型的继承还是很有用的。