Control if a field is unique in a django form - django

Question

First error :I want to check if group_name field value is unique in a modelforms
I tried this code but seems not working as if i put a new value in this field, there is no message but data as not been add to table
Second error: i nether have any message. Maybe, I should not redirect pages ?
my code in views.py:
def group_create(request):
group_form = GroupForm()
if request.method == "POST":
group_form = GroupForm(request.POST)
if group_form.is_valid():
group_name = group_form.cleaned_data.get('group_name')
if Group.objects.filter(group_name=group_name).exists:
messages.error(request, 'this group already exists')
else:
group_form.save()
messages.success(request, 'Group created')
return redirect('group_list')
return render(request, 'imports/group_create.html', {
"group_form": group_form,
})
my model:
class Group(models.Model):
group_id = models.AutoField(primary_key=True)
groupParent_id = models.ForeignKey('self', blank=True, null=True, related_name='Parent', on_delete=models.CASCADE)
group_name = models.CharField(max_length=100, null=False, blank=False, unique=True)
my html:
<div class="container-fluid">
<!-- Code pour message type toaster -->
{% if messages %}
<div class="row">
<div class="col-xs-12">
<ul class="alert" role="alert">
{% for message in messages %}
<p {% if message.tags %} class="{{ message.tags }}"{% endif %}>{{ message }}</p>
{% endfor %}
</ul>
</div>
</div>
{% endif %}
<!-- fin messages -->
<div class="row">
<div class="col-lg-2 col-md-2 col-sm-2 sidebar">
{% block sidebar %}{% endblock %}
</div>
<div class="col-lg-10 col-lg-offset-2 col-md-10 col-md-offset-2 col-sm-10 col-sm-offset-2 content">
{% block heading %}{% endblock %}
{% block page %}{% endblock %}
</div>
</div>
</div>

Answer 1

Django's ModelForm [Django-doc] already validates the uniqness of fields that you marked unique=True [Django-doc], as is specified in the documentation on Interaction with model validation [Django-doc]:
(...)
The model's clean() method will be called before any uniqueness
checks are made. (...).
If the field is not valid, than it will add that error to the group_form.errors [Django-doc]. The reason why you did not see those is that you, regardless whether the form is valid or not, just redirect to a view, and thus the form is discarded.
If you render the form accordingly, it will add extra messages to the fields with errors:
def group_create(request):
if request.method == "POST":
group_form = GroupForm(request.POST)
if group_form.is_valid():
group_form.save()
messages.success(request, 'Group created')
# only in case of success
return redirect('group_list')
else:
group_form = GroupForm()
return render(request, 'imports/group_create.html', {
"group_form": group_form,
})