Django Tutorial - ManyToManyField via a Comma Separated String in Admin & Forms

from django.conf import settings
from django.db import models

# topics.models.py
class TopicManager(models.Manager):
    def create_or_new(self, title):
        title = title.strip()
        qs = self.get_queryset().filter(title__iexact=title)
        if qs.exists():
            return qs.first(), False
        return Topic.objects.create(title=title), True
    
    def comma_to_qs(self, topics_str):
        final_ids = []
        for topic in topics_str.split(','):
            obj, created = self.create_or_new(topic)
            final_ids.append(obj.id)
        qs = self.get_queryset().filter(id__in=final_ids).distinct()
        return qs
        
    
class Topic(models.Model):
    title = models.CharField(max_length=120)
    slug = models.SlugField(blank=True, null=True)
    
    objects = TopicManager()

# posts.models.py
class Post(models.Model):
    user = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.CASCADE)
    title = models.CharField(max_length=120)
    slug = models.SlugField()
    body = models.TextField()
    publish_date = models.DateTimeField(auto_now=False, auto_now_add=False, null=True, blank=True)
    topics = models.ManyToManyField(Topic, blank=True)

# posts.forms.py
from django import forms
from django.utils import timezone

class PostForm(forms.ModelForm):
    topic_str = forms.CharField(label='Topics', widget=forms.Textarea, required=False)
    class Meta:
        model = Post
        fields = [
            "user",
            "title",
            "media",
            "slug",
            "body",
            "topic_str",
            "publish_date"
        ]
    
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        instance = kwargs.get("instance")
        if instance:
            self.fields['topic_str'].initial = ", ".join(x.title for x in instance.topics.all())
    
    def save(self, commit=True, *args, **kwargs):
        instance = super().save(commit=False, *args, **kwargs)
        topics_str = self.cleaned_data.get('topics_str')
        if commit:
            topic_qs = Topic.objects.comma_to_qs(topics_str)
            if not instance.id:
                '''
                This is a new instance.
                '''
                instance.save()
            instance.topics.clear()
            instance.topics.add(*topic_qs)
            instance.save()
        return instance

# posts.admin.py
from django.contrib import admin
from django.db.models import Count
from django.utils.safestring import mark_safe

from topics.models import Topic

from .forms import PostForm

class PostAdmin(admin.ModelAdmin):
    raw_id_fields = ['user']
    list_display = ["title", "publish_date", "topic_list"]
    form = PostForm
    
    def topic_list(self, obj):
        topic_list = ", ".join([f"<a href='/blog/post/?topics__title={x.title}'>{x.title}</a>" for x in obj.topics.all()])
        return mark_safe(topic_list)
    
    def save_model(self, request, obj, form, change):
        topic_str = form.cleaned_data.get('topic_str')
        topic_qs = Topic.objects.comma_to_qs(topic_str)
        if not obj.id:
            obj.save()
        obj.topics.clear()
        obj.topics.add(*topic_qs)
        obj.save()

admin.site.register(Post, PostAdmin)