Why some fields are not created while making migrations? - python-3.x

Question

I have this two models in 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)
When i makemigrations:
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)),
],
),
My Question is: Why Django did not create the fields: "bid" and "user" specified in models.py.

Answer 1

The key to understand this behaviour is the method generate_created_models used when Django tries to figure out the operations needed when generating the migrations. In this method's docs we can read the following:
"""
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).
"""
So for fields that refer to Foreign Keys or Many to Many relationships, Django will try to optimize the operations. The most common options are:
Include the field when creating the model with migrations.CreateModel (as you can see, you have an example in your code for the Bid.user field).
Create the model with migrations.CreateModel without the field. After the creation, Django adds each new field with migrations.AddField (this is what is happening to your AuctionListings.owner, AuctionListings.bid and AuctionListings.watchlist fields, which aren't included while calling migrations.CreateModel in your code).
All in all, an important thing to note is that fields are not created. Actually, models are created and this models contain fields, which can be specified during creation or added later.
With that into account, in your migrations file, just below the lines of code that you posted you should see some operations using migrations.AddField and account for the addition of the fields that are not included in migrations.CreateModel.