INSTALLED_APPS = [
    ...

    'crispy_forms',
]

CRISPY_TEMPLATE_PACK = 'bootstrap4'

from martor.models import MartorField

class Post(models.Model):
    post_subject = models.CharField(max_length=POST_SUBJECT_SIZE, null=True)
    message = MartorField(max_length=MESSAGE_FIELD_SIZE,
                          help_text=f'Maximum length is {MESSAGE_FIELD_SIZE} characters',
                          )
    topic = models.ForeignKey(Topic, on_delete=models.CASCADE,
                              related_name='posts')
    created_by = models.ForeignKey(User, on_delete=models.CASCADE,
                                   related_name='posts')
    created_at = models.DateTimeField(auto_now_add=True)
    updated_by = models.ForeignKey(User, on_delete=models.CASCADE,
                                   related_name='+', null=True)
    updated_at = models.DateTimeField(null=True)
    allowed_editor = models.ManyToManyField(User, blank=True)

    def __str__(self):
            return Truncator(self.post_subject).chars(30)

class PostForm(forms.ModelForm):
    allowed_editor = forms.ModelMultipleChoiceField(widget=forms.CheckboxSelectMultiple(),
        required=False,
        label='',
        help_text='Selection is not required',
        queryset=User.objects.all().order_by('username'))

    class Meta:
        model = Post
        fields = ['allowed_editor', 'post_subject', 'message']
        labels = {'post_subject': 'Post subject'}

@method_decorator(login_required, name='dispatch')
class PostUpdateView(UpdateView):
    model = Post
    form_class = PostForm
    template_name = 'boards/edit_post.html'
    pk_url_kwarg = 'post_pk'
    context_object_name = 'post'

    def get_queryset(self):
        self.topic = get_object_or_404(Topic,
                                       board__pk=self.kwargs.get('board_pk'),
                                       pk=self.kwargs.get('topic_pk'))
        queryset = self.topic.posts.order_by('-updated_at')

    def form_valid(self, form):
        # note if commit=False, then post.save() must be followed by form.save_m2m()
        post = form.save(commit=False)
        post.updated_by = self.request.user
        post.updated_at = timezone.now()
        post.save()
        form.save_m2m()

        self.topic.last_updated = post.updated_at
        self.topic.save()

        topic_url = reverse('topic_posts',
                            kwargs={'board_pk': self.topic.board.pk,
                                    'topic_pk': self.topic.pk},)
        topic_post_url = f'{topic_url}?page={self.topic.get_page_number(post.pk)}'
        return redirect(topic_post_url)

<button class="btn dropdown-toggle btn-primary btn-sm" data-toggle="dropdown"
      >Allowed editors<span class="caret"></span></button>
    <ul class="dropdown-menu">
      {{ form.allowed_editor|as_crispy_field }}
    </ul>

{% extends 'base.html' %}

{% load static %}
{% load crispy_forms_tags %}

{% block title %}Edit post{% endblock %}

{% block breadcrumb %}
<ol class="breadcrumb my-4">
  <li class="breadcrumb-item"><a href="{% url 'home' %}">Home</a></li>
  <li class="breadcrumb-item"><a href="{% url 'boards' %}">Boards</a></li>
  <li class="breadcrumb-item"><a href="{% url 'board_topics' post.topic.board.pk %}">{{ post.topic.board.name }}</a></li>
  <li class="breadcrumb-item"><a href="{% url 'topic_posts' post.topic.board.pk post.topic.pk %}">{{ post.topic.topic_subject }}</a></li>
  <li class="breadcrumb-item active">Edit contribution</li>
</ol>
{% endblock %}

{% block content %}
  <form method="post" class="mb-4" novalidate>
    {% csrf_token %}
    <button type="submit" class="btn btn-primary btn-sm">Save changes</button>
    <button style="margin:1px" class="btn btn-primary btn-sm"
            name="deleted_post_pk" value="{{ post.pk }}"
            onclick="return Validate()">Delete contribtion</button>
    <button class="btn dropdown-toggle btn-primary btn-sm" data-toggle="dropdown"
      >Allowed editors<span class="caret"></span></button>
    <ul class="dropdown-menu">
      {{ form.allowed_editor|as_crispy_field }}
    </ul>
    <a href="{% url 'topic_posts' post.topic.board.pk post.topic.pk %}"
       class="btn btn-outline-secondary btn-sm"
       role="button">Cancel</a>
    <br></br>
    {{ form.post_subject|as_crispy_field }}
    {{ form.message|as_crispy_field }}
  </form>
  <br>
  <br>
{% endblock %}

{% block javascript %}
  <script src="{% static 'js/board.js' %}"></script>
{% endblock %}