详解Python的Django框架中Manager方法的使用
在语句Book.objects.all()中，objects是一个特殊的属性，需要通过它查询数据库。 在第5章，我们只是简要地说这是模块的manager 。现在是时候深入了解managers是什么和如何使用了。
总之，模块manager是一个对象，Django模块通过它进行数据库查询。 每个Django模块至少有一个manager，你可以创建自定义manager以定制数据库访问。
下面是你创建自定义manager的两个原因： 增加额外的manager方法，和/或修manager返回的初始QuerySet。
增加额外的Manager方法
增加额外的manager方法是为模块添加表级功能的首选办法。
例如,我们为Book模型定义了一个title_count()方法，它需要一个关键字，返回包含这个关键字的书的数量。 （这个例子有点牵强，不过它可以说明managers如何工作。）
# models.py
from django.db import models
# ... Author and Publisher models here ...
**class BookManager(models.Manager):**
**def title_count(self, keyword):**
**return self.filter(title__icontains=keyword).count()**
class Book(models.Model):
title = models.CharField(max_length=100)
authors = models.ManyToManyField(Author)
publisher = models.ForeignKey(Publisher)
publication_date = models.DateField()
num_pages = models.IntegerField(blank=True, null=True)
**objects = BookManager()**
def __unicode__(self):
return self.title有了这个manager，我们现在可以这样做：
>>> Book.objects.title_count('django')
4
>>> Book.objects.title_count('python')
18
下面是编码该注意的一些地方：
- 我们建立了一个BookManager类，它继承了django.db.models.Manager。这个类只有一个title_count()方法，用来做统计。 注意，这个方法使用了self.filter()，此处self指manager本身。
- 我们把BookManager()赋值给模型的objects属性。 它将取代模型的默认manager（objects）如果我们没有特别定义，它将会被自动创建。 我们把它命名为objects，这是为了与自动创建的manager保持一致。
为什么我们要添加一个title_count()方法呢?是为了将经常使用的查询进行封装，这样我们就不必重复编码了。
修改初始Manager QuerySets
manager的基本QuerySet返回系统中的所有对象。 例如,`` Book.objects.all()`` 返回数据库book中的所有书本。
我们可以通过覆盖Manager.get_query_set()方法来重写manager的基本QuerySet。 get_query_set()按照你的要求返回一个QuerySet。
例如,下面的模型有* 两个* manager。一个返回所有对像，另一个只返回作者是Roald Dahl的书。
from django.db import models
**# First, define the Manager subclass.**
**class DahlBookManager(models.Manager):**
**def get_query_set(self):**
**return super(DahlBookManager, self).get_query_set().filter(author='Roald Dahl')**
**# Then hook it into the Book model explicitly.**
class Book(models.Model):
title = models.CharField(max_length=100)
author = models.CharField(max_length=50)
# ...
**objects = models.Manager() # The default manager.**
**dahl_objects = DahlBookManager() # The Dahl-specific manager.**
在这个示例模型中,Book.objects.all()返回了数据库中的所有书本,而Book.dahl_objects.all()只返回了一本. 注意我们明确地将objects设置成manager的实例，因为如果我们不这么做，那么唯一可用的manager就将是dah1_objects。
当然,由于get_query_set()返回的是一个QuerySet对象，所以我们可以使用filter()，exclude()和其他一切QuerySet的方法。 像这些语法都是正确的：
Book.dahl_objects.all() Book.dahl_objects.filter(title='Matilda') Book.dahl_objects.count()
这个例子也指出了其他有趣的技术： 在同一个模型中使用多个manager。 只要你愿意，你可以为你的模型添加多个manager()实例。 这是一个为模型添加通用滤器的简单方法。
例如:
class MaleManager(models.Manager):
def get_query_set(self):
return super(MaleManager, self).get_query_set().filter(sex='M')
class FemaleManager(models.Manager):
def get_query_set(self):
return super(FemaleManager, self).get_query_set().filter(sex='F')
class Person(models.Model):
first_name = models.CharField(max_length=50)
last_name = models.CharField(max_length=50)
sex = models.CharField(max_length=1, choices=(('M', 'Male'), ('F', 'Female')))
people = models.Manager()
men = MaleManager()
women = FemaleManager()
这个例子允许你执行`` Person.men.all()`` ，`` Person.women.all()`` ，`` Person.people.all()`` 查询，生成你想要的结果。
如果你使用自定义的Manager对象，请注意，Django遇到的第一个Manager(以它在模型中被定义的位置为准)会有一个特殊状态。 Django将会把第一个Manager 定义为默认Manager ，Django的许多部分(但是不包括admin应用)将会明确地为模型使用这个manager。 结论是，你应该小心地选择你的默认manager。因为覆盖get_query_set() 了，你可能接受到一个无用的返回对像，你必须避免这种情况。
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