Django Add Field Error to Non-Model Form Field - django

Question

Can anyone help me understand how to properly send a field error back to a non-model form field in django that is not using the standard django form validation process? Rendering the form again with error and user data still entered for correction and resubmission?
Example html for a simple username field that is validated on the model level:
<!--form-->
<form id="profile" class="small" method="POST" action="{% url 'profile' %}">
{% csrf_token %}
<!--Username-->
<label for="username">Username <span style="font-style: italic;">(create a unique display name that will appear to other users on the site)</span></label>
<div class="input-group mb-3">
<div class="input-group-prepend">
<span class="input-group-text" id="username">#</span>
</div>
<input type="text" class="form-control" placeholder="Username" aria-label="Username" aria-describedby="username" name="username" value="{% if profile and profile.username is not None %}{{ profile.username }}{% endif %}">
</div>
<button type="submit" class="btn btn-primary" name="profile_form" value="profile_form">Save</button>
</form>
View
class ProfileView(View):
def get(self, request, *args, **kwargs):
# get request...
def post(self, request, *args, **kwargs):
if request.method == "POST":
# check if profile_form submitted
if 'profile_form' in request.POST:
# get user form data
profile_data = request.POST.dict()
# get current user profile
user_profile = Profile.objects.get(my_user=request.user)
# check username entry against current
if user_profile.username == profile_data['username']:
messages.success(request, "This is the current user.")
else:
try:
# try to save the new username
user_profile.username = profile_data['username']
user_profile.save(update_fields=['username'])
messages.success(request, "Success: Username was updated.")
except:
# unique constraint error on username
# ERROR PROCESSING NEEDED
# Need to send error to form for user to correct and resubmit???
# return get request to process any updated data
return HttpResponseRedirect(reverse('profile'))

Answer 1

You should use a form class that takes care of the form data and it could also repopulate the data.
forms.py
class ProfileForm(forms.Form):
username = forms.CharField(
required=True,
label="Username",
help_text="(create a unique display name that will appear to other users on the site)",
)
views.py
class ProfileView(View):
def get(self, request, *args, **kwargs):
# get request...
def post(self, request, *args, **kwargs):
if request.method == "POST":
form = ProfileForm(request.POST)
if form.is_valid():
username = form.cleaned_data["username"]
user_profile = Profile.objects.get(my_user=request.user)
if user_profile.username == username:
messages.success(request, "This is the current user.")
else:
try:
user_profile.username = username
user_profile.save(update_fields=["username"])
messages.success(request, "Success: Username was updated.")
except IntegrityError:
messages.error(request, "Error message which should be displayed")
return render(
request, "your_form.html", {"form": form}
)
return HttpResponseRedirect(reverse('profile'))