from django.db import models
# 自定义模型管理器类
class BookManager(models.Manager):
  #自定义模型管理器中的方法
  def title_count(self, keyword):
    return self.filter(title_icountains=keyword).count()
class Book(models.Model):
  title = models.CharField(max_length=100)
  authors = models.ManyToManyField(Author)
  ...
  objects = BookManager()
  def __str__(self):
    return self.title

> Books.objects.title_count('django')  #这是我们自定义的manager中的查询方法

> Books.objects.filter(title__icontains='django').count()  # 默认的查询方法依然可用

from django.db import models

#首先,定义一个Manager的子类
class DahlBookManager(models.Manager):
  def get_queryset(self):
    return super(DahlBookManager, self).get_queryset().filter(author='Roald Dahl')


# 然后,将它显式地插入到Book模型中
class Book(models.Model):
  title = models.CharField(max_length=100)
  author = models.CharField(max_length=50)
  ...
  objects = models.Manager()  # 默认Manager
  dahl_objects = DahlBookManager()  # 自定义的特殊Manager

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):
    # Return the person's full name
    return f'{self.first_name} {self.last_name}'
  full_name = property(_get_full_name)  # 将类方法包装为属性

> p = Person.objects.get(first_name='Barack', last_name='Obama')
> p.birth_date
datetime.date(1961, 8, 4)
> p.baby_boomer_status()
'Baby boomer'
> p.full_name # 注意这不是一个方法 -- 它被视为一个属性
'Barack Obama'

from django.db import models

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

  def save(self, *args, **kwargs):
    do_something()
    super(Blog, self).save(*args, **kwargs)  #Call the "real" save() method.
    do_something_else()

from django.db import models
class Blog(models.Model):
  name = models.CharField(max_length=100)
  tagline = models.TextField()
  def save(self, *args, **kwargs):
    if self.name == 'Yoko Ono's Blog':
      return  # Yoko shall never have her own blog!
    else:
      super(Blog, self).save(*args, **kwargs)  #Call the "real" save() method

import detetime
from django.core.exceptions import ValidationError
from django.db import models

class Article(models.Model):
  ...
  def clean(self):
    # Don't allow draft entries to have a pub_date
    if self.status == 'draft' and self.pub_date is not done:
      raise ValidationEroor('Draft entries may not have a publication date')
    #Set the pub_date for published items if it hasn't been set already
    if self.status == 'published' and self.pub_date is None:
      self.pub_date = datetime.date.today()

from django.core.exceptions import ValidationError, NON_FIELD_ERRORS
try:
  article.full_clean()
except ValidationError as e:
  non_field_errors = e.message_dict[NON_FIELD_ERRORS]

class Article(models.Model):
  ...
  def clean(self):
    # Don't allow draft entries to have a pub_date.
    if self.status == 'draft' and self.pub_date is not None:
      raise ValidationError({'pub_date': 'Draft entries may not have a publication date.'})
    ...