Django change button status

Hi,

I search through internet and review many apps but still don't have answer maybe on this forum someone help me.

I try to create changing button after user join to Goal.

I have work buttons, but i don't know how to change this in template.

I try to do this with boolean field, when user click "Join", boolean field change to True, and then in templates:

{% if goal.joined %}
<Delete join>
{% else %}
<join to goal>
{% endif %}

For now user can click many times on button and create many "join" objects in one goal.
Now i know that i need to create variable in view that checks if the current user has joined the goal and add it to the context. But I don't know how. Im still newby.

class Goal(models.Model, Activity):
    title = models.CharField(max_length=255, verbose_name='Tytuł')
    image = models.ImageField(upload_to='goals', verbose_name='Tło')
    body = HTMLField(verbose_name='Treść')
    tags = TaggableManager()
    created_at = models.DateTimeField(auto_now_add=True)
    author = models.ForeignKey(User, on_delete=models.CASCADE)
    slug = AutoSlugField(populate_from='title')

    def __str__(self):
        return self.title

    class Meta:
        verbose_name = 'Cele'
        ordering = ['-created_at']

    def get_absolute_url(self):
        return reverse('goaldetail', args=[str(self.slug)])

    @property
    def activity_actor_attr(self):
        return self.author

    def add_user_to_list_of_attendees(self, user):
        registration = Joined.objects.create(user = user,
                                                    goal = self,
                                                    created_at = timezone.now())

    def remove_user_from_list_of_attendees(self, user):
        registration = Joined.objects.get(user = user, goal = self)
        registration.delete()

class Joined(models.Model, Activity):
    goal = models.ForeignKey(Goal, on_delete=models.CASCADE, related_name='joined')
    user = models.ForeignKey(User, on_delete=models.CASCADE, related_name='joined_users')
    created_at = models.DateTimeField(auto_now_add=True)
    joined = models.BooleanField(default=False)

    def save(self, *args, **kwargs):
        if self.id is None and self.created_at is None:
            self.created_at = datetime.datetime.now()
        self.joined = True
        super(Joined, self).save(*args, **kwargs)

    @property
    def activity_actor_attr(self):
        return self.user

Views:

def joined_add(request, pk):
    this_goal = Goal.objects.get(pk=pk)
    this_goal.add_user_to_list_of_attendees(user=request.user)
    return redirect('goaldetail', slug=this_goal.slug)

def joined_delete(request, pk):
    this_goal = Goal.objects.get(pk=pk)
    this_goal.remove_user_from_list_of_attendees(request.user)
    return redirect('goaldetail', slug=this_goal.slug)

def goaldetail(request, slug):
    goal = get_object_or_404(Goal, slug=slug)
    return render(request, 'goals/detail.html',
                {'goal': goal})

Path:

path('joined/<int:pk>', views.joined_add, name='joined_add'),
path('joined-delete/<int:pk>', views.joined_delete, name='joined_delete'),