Django Добавление комментария к посту. Ошибка "Comment.username" must be a "User" instance

Хочу сделать так, чтобы залогиненный пользователь мог оставлять комментарии к постам. Знаю, что ошибка в PostDetailView, но не получается исправить. Объясните пожалуйста, что я делаю не так.

views:

class PostDetailView(DetailView):
    model = Post
    template_name = 'base/post_detail.html'

    def get_context_data(self, **kwargs):
        context = super().get_context_data(**kwargs)
        pk = self.kwargs["pk"]

        post = get_object_or_404(Post, pk=pk)
        comments = post.comment_set.all().filter(active=True)
       
        context['comments'] = comments
        context['form'] = CommentForm()
        return context
    
    def post(self, request, *args, **kwargs):
        form = CommentForm(request.POST)
        self.object = self.get_object()
        context = super().get_context_data(**kwargs)

        post = Post.objects.filter(id=self.kwargs['pk'])[0]
        comments = post.comment_set.all()
        username = self.request.user.username
        context['post'] = post
        context['username'] = username
        context['comments'] = comments
        context['form'] = form

        if form.is_valid():
            
            content = form.cleaned_data['content']

            comment = Comment.objects.create(
               username=username, content=content, post=post
            )

            form = CommentForm()
            context['form'] = form
            
            return self.render_to_response(context=context)

        return self.render_to_response(context=context)

models:

class Post(models.Model):
    title = models.CharField(max_length=100)
    content = models.TextField()
    date_posted = models.DateTimeField(default=timezone.now)
    author = models.ForeignKey(User, on_delete=models.CASCADE)

    def __str__(self) -> str:
        return f'{self.author}: {self.title}'
    
    def get_absolute_url(self):
        return reverse("base:post-detail", kwargs={"pk": self.pk})
    
    class Meta:
        ordering = ['-date_posted']
        indexes = [
            models.Index(fields=['-date_posted'])
        ]
    

class Comment(models.Model):
    post = models.ForeignKey(Post, on_delete=models.CASCADE)
    username = models.ForeignKey(User, on_delete=models.CASCADE)
    content = models.TextField()
    created = models.DateTimeField(auto_now_add=True)
    updated = models.DateTimeField(auto_now=True)
    active = models.BooleanField(default=True)

    class Meta:
        ordering = ['created']
        indexes = [
            models.Index(fields=['created'])
        ]

    def __str__(self) -> str:
        return f'Comment by {self.username} on {self.post}'

forms:

from .models import Comment
from django import forms

class CommentForm(forms.ModelForm):
    class Meta:
        model = Comment
        fields = ['content']

post_detail.html:

{% extends "base/base.html" %}

{% block content %}
    <article class="media content-section">
      <img class="rounded-circle article-img" src='{{ object.author.profile.image.url }}'>
        <div class="media-body">
          <div class="article-metadata">
            <a class="mr-2" href="#">{{ object.author }}</a>
            <small class="text-muted">{{ object.date_posted|date:"F d, Y" }}</small>
            {% if object.author == user %}
              <div>
                <a class="btn btn-secondary btn-sm mt-1 mb-1" href="{% url "base:post-update" object.id %}">Update</a>
                <a class="btn btn-danger btn-sm mt-1 mb-1" href="{% url "base:post-delete" object.id %}">Delete</a>
              </div>
            {% endif %}
          </div>
          <h2 class="article-title">{{ object.title }}</h2>
          <p class="article-content">{{ object.content }}</p>
        </div>
      </article>
        <!-- Form to leave comment -->
    <div class="row mt-5">
      <div class="col-lg-6 offset-lg-3">
        <h3>Leave a comment</h3>
        <form method='POST'>
          {% csrf_token %}
          <div class="form-group">
            <span class="ml-2"></span>{{ form.content.label_tag }}
            <textarea class="form-control {% if form.content.errors %}is-invalid{% endif %}" id="id_content"
              name="content" rows="4">{{ form.content.value|default:'' }}</textarea>
          </div>
          <button class="btn btn-primary ml-2" type="submit">Reply</button>
        </form>
      </div>
    </div>
  </div>
      <!-- List of comments -->
  {% if comments %}
  <div class="row mt-5">
    <div class="col-lg-6 offset-lg-3">
      Comment{{ comments.count|pluralize }}
      <span class="badge badge-dark ml-2">{{ comments.count }}</span>
    </div>
    {% for comment in comments %}
    <div class="col-lg-6 offset-lg-3 mt-2">
      <div class="card p-2">
        <div class="row">
          <div class="col-12">
            <img class="rounded-circle article-img" src='{{ object.author.profile.image.url }}'>
            <strong>{{ comment.username }}</strong>
          </div>
          <div class="col-12">
            <p class="m-1 mt-3">{{ comment.content }}</p>
            <p class="text-right text-muted"><small>{{ comment.created }}</small></p>
          </div>
        </div>
      </div>
    </div>
    {% endfor %}
  </div>
  {% endif %}

  
{% endblock content %}