Django的檢索相關對象

Question

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") 

它打印

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

雖然我希望它在同一個訂單時，您將它們添加打印爲我加了他們這樣

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

Answer 1

你失去聯絡您添加它們的順序，因爲它只設置Book的外鍵。

你可以做的就是添加一個DateTimeField（）您的圖書，並設置它，當你將它添加到出版商和ORDER_BY它。

類似：

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) 

的，當你添加它

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() 

，當你打印它：

p.b.order_by("_published_at") 

編輯： 你對你所需要的ManyToManyField問題定製的「通過」型號 看到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'] 

你也不能使用add()添加新的發佈商，您需要定義新車型這樣做。

所以要加出版商P3預訂B7你需要做的

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

他們應該自動現在