查詢不在表中的記錄

Question

繼續以Django為例：

from django.db import models

class Publication(models.Model):
    title = models.CharField(max_length=30)

class Article(models.Model):
    headline = models.CharField(max_length=100)
    publications = models.ManyToManyField(Publication)    

我如何獲得所有沒有出版物的文章？

Article.objects.filter(...) 

Answer 1

將publications指定為“無”：

Article.objects.filter(publications=None)

例：

>>> p1 = Publication.objects.create(title='1')
>>> p2 = Publication.objects.create(title='2')
>>> a1 = Article.objects.create(headline='a')
>>> a2 = Article.objects.create(headline='b')
>>> a3 = Article.objects.create(headline='c')
>>> a1.publications.add(p1)
>>> a1.publications.add(p2)
>>> a3.publications.add(p1)
>>> Article.objects.filter(publications=None)
[<Article: Article object>]
>>> _[0].headline
u'b'

Answer 2

Article.objects.filter(publications=None)

在“文章”模型中定義“ blank = True with Publications”字段，否則它將不允許您創建沒有出版物的文章。

publications = models.ManyToManyField(Publication, blank=True)