[Django]-How to modify a models who's already migrated in Database?

First you need to edit your models.py

# models.py
class Post(models.Model):
    name = models.CharField(max_length=100)
    email = models.CharField(max_length=100)
    gender = models.CharField(max_length=8, choices=THE_GENDER)
    number = models.CharField(max_length=100)
    author = models.ForeignKey(User, on_delete=models.CASCADE) # add new field

    def __str__(self):
        return self.name

Since you added new field author, you have to run makemigrations command and then the migrate command.
During migrations, you may be asked to provide a default value for author field for existing Post instances. In that time, you can “interactively” provide values via Django shell. Any valid PK of User instances will be a valid input during the time. FYI: This is a onetime process

After that, change your home(...) view as,

from django.contrib.auth.decorators import login_required


@login_required
def home(request):
    form = post_form(request.POST or None)
    if form.is_valid():
        instance = form.save(commit=False)
        instance.auther = request.user
        instance.save()

    context = {
        "form": form
    }

    return render(request, "sms/home.html", context)

Note: You should use @login_required(...) decorator to get the authenticated user in request.user attribute.

Your migration is tricky, because you want every Post related to a user. To workaround this you can do the following:

Add your new field with null=True
models.ForeignKey(User, on_delete=models.CASCADE, null=True)
then manage.py makemigrations
then you have to decide what to do. Either (1) add authors to your existing posts by hand, or (2) delete them all

manage.py shell
from x.models import Post
from django.contrib.auth.models import User

# 1
user = User.objects.get(id=1) # get your user
Post.objects.all().update(author=user)

# or 2
Post.objects.all().delete()

then you can finally set your authors like you want them:
models.ForeignKey(User, on_delete=models.CASCADE)
again manage.py makemigrations

See the comments for more information

You can handle this issue in different ways.

If there is no data, you can easily add the new field and run your migrations. You should be fine

If you already have posts, you can allow the field to be nullable, then run your migrations before going to populate it.

A more intuitive way of doing this especially if you already have data is to assign a user to the post while running the migrations.

First create a migration with the author field allowed to be null, then

author = models.ForeignKey(User, null=True, on_delete=models.CASCADE)

manage.py makemigrations
manage.py migrate

after that, remove the null attribute.

author = models.ForeignKey(User, on_delete=models.CASCADE)

manage.py makemigrations

open the migration file now generated which will be which you will modify with a custom method like below and run migrate after that.

from django.db import migrations, models


# create custom method
def add_author(apps, schema_editor):
    User = apps.get_model('auth', 'User')  
    Post = apps.get_model('something', 'Post')  
    posts = Post.objects.all()
    author = User.object.get(id=1) # whichever user you want to use as the author
    for post in posts:
        post.author = author
        post.save()

class Migration(migrations.Migration):

    dependencies = [
        ('posts', '00000000'),
    ]

    operations = [
        migrations.RunPython(add_author), # this will run the method above
        migrations.AlterField(
            model_name='post',
            name='author',
            field=models.ForeignKeyField...,
        ),

    ]

check this link for a more detailed explanation of how this works