python之django 检索相关对象

class Publisher(models.Model): 
    name = models.CharField(max_length=300, null=True, blank=True) 
class Book(models.Model): 
    name = models.CharField(max_length=300, null=True, blank=True) 
    publisher = models.ManyToManyField(Publisher,related_name='b', null=True) 
    _published_at = models.DateTimeField() 
from polls.models import Book, Publisher 
p1=Publisher.objects.get(id=1) 
p1.b.clear() 
p1.save() 
b6=Book.objects.get(id=6) 
p1.b.add(b6) 
b6._published_at = datetime.now() 
p1.save() 
b5=Book.objects.get(id=5) 
p1.b.add(b5) 
b5._published_at = datetime.now() 
p1.save() 
b7=Book.objects.get(id=7) 
p1.b.add(b7) 
b7._published_at = datetime.now() 
p1.save() 
 
p2 = Publisher.objects.get(id=2) 
p2.b.clear() 
p2.save() 
b7=Book.objects.get(id=7) 
p2.b.add(b7) 
b7._published_at = datetime.now() 
p2.save() 
b6=Book.objects.get(id=6) 
p2.b.add(b6) 
b6._published_at = datetime.now() 
p2.save() 
b5=Book.objects.get(id=5) 
p2.b.add(b5) 
b5._published_at = datetime.now() 
p2.save() 
 
print p1.b.order_by("_published_at") 
print p2.b.order_by("_published_at") 

打印

[Book: b7, Book: b6, Book: b5] 
[Book: b7, Book: b6, Book: b5] 

虽然我希望它以与我这样添加它们相同的顺序打印

[Book: b6, Book: b5, Book: b7] 
[Book: b7, Book: b6, Book: b5] 

请您参考如下方法:

当您添加它们时，您会忘记添加它们的顺序，因为它只设置 Book 的外键。

您可以做的是将 DateTimeField() 添加到您的 Book 并在将其添加到出版商并按其排序时设置它。

类似于:

class Book(models.Model): 
    name = models.CharField(max_length=300, null=True, blank=True) 
    _published_at = models.DateTimeField() 
    publisher = models.ForeignKey(Publisher,related_name='b', null=True) 

添加的时候

p=Publisher.objects.get(id=2) 
p.b.clear() 
p.save() 
b6=Book.objects.get(id=6) 
p.b.add(b6) 
b6._published = datetime.now() 
b6.save() 
p.save() 
b5=Book.objects.get(id=5) 
p.b.add(b5) 
b5._published = datetime.now() 
b5.save() 
p.save() 
b7=Book.objects.get(id=7) 
p.b.add(b7) 
b7._published = datetime.now() 
b7.save() 
p.save() 

打印时:

p.b.order_by("_published_at") 

编辑: 对于您关于 ManyToManyField 的问题，您需要一个自定义的“through”模型 见https://docs.djangoproject.com/en/dev/topics/db/models/#extra-fields-on-many-to-many-relationships

class Publisher(models.Model): 
    name = models.CharField(max_length=300, null=True, blank=True) 
 
class Book(models.Model): 
    name = models.CharField(max_length=300, null=True, blank=True) 
    publisher = models.ManyToManyField(Publisher,related_name='b',through="Publication",null=True) 
    _published_at = models.DateTimeField() 
 
class Publication(models.Model): 
    book = models.ForeignKey(Book, on_delete=models.CASCADE) 
    publisher = models.ForeignKey(Publisher, on_delete=models.CASCADE) 
    _published_at = models.DateField() 
 
    class Meta: 
        ordering = ['_published_at'] 

此外，您不能使用 add() 添加新的发布者，您需要定义新的模型才能这样做。

所以要将 Publisher p3 添加到 book b7，你需要做

Publication.objects.create(book=b7, publisher=p3, _published_at=datetime.now()) 

他们现在应该自动订购了