django db models探索

一、django db models结构

django db models 属于django db的一部分。主要包括四部分：

1. models

2. querySet

3. query

4. manager

 

 

1. models。

models定义在djang/db/models/base.py中，一些比较重要的类:

ModelBase	ModelBase是一个元类，继承于type，通过覆盖__new__方法动态创建Model类型，在Model中通过__metaclass__使用它。
ModelState	A class for storing instance state
Model	Mode就是具体的django models，一般我们自定义的模型都需要继承于它，比如： from django.db import models class Message(models.Model)： recipient = models.ForeignKey(User, verbose_name=u"收件人", related_name="message_recipients_set", db_index=True)

 

2.querySet

querySet定义在django/db/models/query.py,ORM的核心API定义在这里面 , 一些比较重要的类:

QuerySet	querySet是最核心的类，既表示返回的结果又能在结果上继续执行查询。当执行 Message.objects.filter(id=12020231)  返回的就是QuerySet
ValuesQuerySet	调用querySet的values()返回此对象 m = Message.objects.filter(id=12020231) m.values() 输出： [{'status': 0, 'category': 12, 'buyable': 0, 'photo_id': 309867L...}]
ValuesListQuerySet
DateQuerySet
EmptyQuerySet
RawQuerySet
get_cached_row()
insert_query()

 

 QuerySet定义了很多常用的查询接口：

1. iterator
2. aggregate
3. count
4. get
5. create
6. get_or_create
7. latest
8. delete
9. update
10. exists
11. values
12.  filter
13. exclude
14. using

具体的查询SQL是委托给self.query产生的，请看filter方法的具体实现：

    def _filter_or_exclude(self, negate, *args, **kwargs):
        if args or kwargs:
            assert self.query.can_filter(), \
                    "Cannot filter a query once a slice has been taken."

        clone = self._clone()
        if negate:
            clone.query.add_q(~Q(*args, **kwargs))
        else:
            clone.query.add_q(Q(*args, **kwargs))
        return clone 

3. query定义在djabgo/db.models/sql/query.py中，负责为QuerySets创建sql语句，里面定义了两个重要的类型

Query  A single SQL query.

RawQuery A single raw SQL query

Query覆盖了str方法，返回具体的SQL语句

    def __str__(self):
        """
        Returns the query as a string of SQL with the parameter values
        substituted in.

        Parameter values won't necessarily be quoted correctly, since that is
        done by the database interface at execution time.
        """
        sql, params = self.get_compiler(DEFAULT_DB_ALIAS).as_sql()
        return sql % params

所以通过

Message.objects.filter(id=121201).query

 可以打印出SQL语句。

4.manager定义在django/db/models/manager.py中：

Manager

ManagerDescriptor

EmptyManager

Manager定义了常用的ORM api:

1. get_query_set()
2. all()
3. count()
4. distinct()
5. get()
6. get_or_create()
7. create()
8. filter()
9. aggregate()
10. order_by()
11. values()
12. update()
13. reverse()
14. exists()

所有这些API都是通过调用get_query_set()委托给querySet实现的。
 

四、一些疑问和答案

1.django如何查看执行的sql语句?

 

from django.db import connection
print connection.queries

 

 2.djangor如何拦截到update操作？

from django.db.models import signals
signals.class_prepared.connect(setup_join_cache)

from django.dispatch import Signal
post_update = Signal(providing_args=['instance'])
post_create = Signal(providing_args=['instance'])

3. models对象能直接new吗？

可以:

k = Message()
k.photo_id = 309867
print k.photo

 

4. 通过filter()执行查询是在什么情况下执行SQL语句的，多次filter()会多次查询吗？

不会，请参考django文档对query_set的解释：

https://docs.djangoproject.com/en/dev/topics/db/queries/

QuerySets are lazy – the act of creating a QuerySet doesn’t involve any database activity. You can stack filters together all day long, and Django won’t actually run the query until the QuerySet is evaluated. Take a look at this example:

>>> q = Entry.objects.filter(headline__startswith="What")
>>> q = q.filter(pub_date__lte=datetime.date.today())
>>> q = q.exclude(body_text__icontains="food")
>>> print(q)

 Though this looks like three database hits, in fact it hits the database only once, at the last line (print(q)). In general, the results of a QuerySet aren’t fetched from the database until you “ask” for them. When you do, the QuerySet is evaluated by

 

 

 参考：

http://blog.xiaket.org/2009/pro-django-ch2-notes-1.html
http://blog.jobbole.com/21351/
http://jeffelmore.org/2010/09/25/smarter-caching-of-django-querysets/