1. Model method vs Model manager
Model method define custom methods on a model to add custom “row-level” functionality to your objects.
Whereas Manager methods are intended to do “table-wide” things, model methods should act on a particular model instance.
Model method：
一个例子:
from django.db import models
class Person(models.Model):
first_name = models.CharField(max_length=50)
last_name = models.CharField(max_length=50)
birth_date = models.DateField()
def baby_boomer_status(self):
"Returns the person's baby-boomer status."
import datetime
if self.birth_date < datetime.date(1945, 8, 1):
return "Pre-boomer"
elif self.birth_date < datetime.date(1965, 1, 1):
return "Baby boomer"
else:
return "Post-boomer"
def _get_full_name(self):
"Returns the person's full name."
return '%s %s' % (self.first_name, self.last_name)
full_name = property(_get_full_name)
这里可以看到baby_boomer_status以及_get_full_name都是针对于一个Model的instance的
Manager methods
from django.db import models
class PollManager(models.Manager):
def with_counts(self):
from django.db import connection
cursor = connection.cursor()
cursor.execute("""
SELECT p.id, p.question, p.poll_date, COUNT(*)
FROM polls_opinionpoll p, polls_response r
WHERE p.id = r.poll_id
GROUP BY p.id, p.question, p.poll_date
ORDER BY p.poll_date DESC""")
result_list = []
for row in cursor.fetchall():
p = self.model(id=row[0], question=row[1], poll_date=row[2])
p.num_responses = row[3]
result_list.append(p)
return result_list
class OpinionPoll(models.Model):
question = models.CharField(max_length=200)
poll_date = models.DateField()
objects = PollManager()
这里可以看到with_counts其实就是执行了一系列的SQL操作。是针对整个table的
也可以继承原有的models.Manager，修改原有的QuerySet
class DahlBookManager(models.Manager):
def get_queryset(self):
return super(DahlBookManager, self).get_queryset().filter(author='Roald Dahl')
2. Model Permission
Django的permission系统即可以针对一个Model,也可以针对一个Model的Instance.
django.contrib.auth.models中有User这个Model.其中有一个ManyToMany relationship to django.contrib.auth.models.Permission的field: user_permissions
class models.User中有
has_perm(perm,object=None)
如果这里的object被传入，那么这个函数将检查这个object有没有这个permission.
3. Custom model field
首先要明白什么是model的field,django官方文档说的很好:
The simplest way to think of a model field is that it provides a way to take a normal Python object – string, boolean, datetime, or something more complex like Hand – and convert it to and from a format that is useful when dealing with the database (and serialization, but, as we’ll see later, that falls out fairly naturally once you have the database side under control).
就是一个interface或者channel，专门处理python本身的对象和database数据之间的转换