为什么在迁移时没有创建一些字段?

Question

我有这两个模型在models.py

class Bid(models.Model):
bid = models.IntegerField(default = 0)
user = models.ForeignKey(User, on_delete = models.CASCADE, related_name = "bid")
def __str__(self):
return f"Bid of {self.bid} from {self.user}"
class AuctionListings(models.Model):
name_of_item = models.CharField(max_length=32)    
description = models.CharField(max_length=400)    
owner = models.ForeignKey(User, on_delete = models.CASCADE, related_name="auctionlistings", default = None)**    
bid = models.ForeignKey(Bid, on_delete = models.CASCADE, related_name = "auctionlistings", default = None)**    
is_closed = models.BooleanField(default=False, blank=True, null=True)
url = models.CharField(max_length=800)    
watchlist = models.ManyToManyField(User, blank=True, related_name="watch_listings")    
category = models.CharField(max_length=50)

当我做迁移时:

operations = [
migrations.CreateModel(
name='AuctionListings',
fields=[
('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
('name_of_item', models.CharField(max_length=32)),
('description', models.CharField(max_length=400)),
('is_closed', models.BooleanField(blank=True, default=False, null=True)),
('url', models.CharField(max_length=800)),
('category', models.CharField(max_length=50)),
],
),
migrations.CreateModel(
name='Bid',
fields=[
('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
('bid', models.IntegerField(default=0)),
('user', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='bid', to=settings.AUTH_USER_MODEL)),
],
),

我的问题是:为什么Django没有创建字段:"bid"one_answers";user"指定于models.py.

Answer 1

理解这种行为的关键是generate_created_models方法，当Django试图找出生成迁移时所需的操作时使用。在这个方法的文档中，我们可以读到以下内容:

"""
Find all new models (both managed and unmanaged) and **make create
operations for them as well as separate operations to create any
foreign key or M2M relationships (these are optimized later, if
possible)**.
Defer any model options that refer to collections of fields that might
be deferred (e.g. unique_together, index_together).
"""

因此，对于引用外键或多对多关系的字段，Django将尝试优化操作。最常见的选项是:

• 在使用migrations.CreateModel创建模型时包含该字段(如您所见，您的代码中有Bid.user字段的示例)。
• 创建不带字段的migrations.CreateModel模型。创建之后，Django用migrations.AddField添加每个新字段(这就是你的AuctionListings.owner,AuctionListings.bid和AuctionListings.watchlist字段发生的情况，它们在你的代码中调用migrations.CreateModel时不包括在内)。

总而言之，需要注意的重要一点是，字段不会被创建。实际上，模型是创建的，并且模型包含字段，这些字段可以在创建期间指定或稍后添加。

考虑到这一点，在您的迁移文件中，就在您发布的代码行下面，您应该看到一些使用migrations.AddField的操作，并说明migrations.CreateModel中未包含的字段的添加。