Django CreateView: How to perform action upon save

Question

I'm using a custom CreateView (CourseCreate) and UpdateView (CourseUpdate) to save and update a Course. I want to take an action when the Course is saved. I will create a new many-to-many relationship between the instructor of the new course and the user (if it doesn't already exist).

So, I want to save the Course as course, and then use course.faculty to create that new relationship. Where is the best place to make this happen?

I'm trying to do this in form_valid in the views, but I'm getting errors when trying to access form.instance.faculty bc the course isn't created yet (in CourseCreate). The error message is like:

"Course: ..." needs to have a value for field "course" before this many-to-many relationship can be used.

It's also not working in CourseUpdate. The Assists relationship is not created. Should I be trying this in the Form? But I'm not sure how to get the user info to the Form. Thank you.

models.py

class Faculty(models.Model):
    last_name = models.CharField(max_length=20)

class Course(models.Model):
    class_title = models.CharField(max_length=120)
    faculty = models.ManyToManyField(Faculty)

class UserProfile(models.Model):
    user = models.OneToOneField(User)
    faculty = models.ManyToManyField(Faculty, through='Assists')

class Assists(models.Model):
    user = models.ForeignKey(UserProfile)
    faculty = models.ForeignKey(Faculty)

views.py

class CourseCreate(CreateView):
    model = Course
    template_name = 'mcadb/course_form.html'
    form_class = CourseForm
    def form_valid(self, form):
        my_course = form.instance
        for f in my_course.faculty.all():
            a, created = Assists.objects.get_or_create(user=self.request.user.userprofile, faculty=f)
        return super(CourseCreate, self).form_valid(form)

class CourseUpdate(UpdateView):
    model = Course
    form_class = CourseForm
    def form_valid(self, form):
        my_course = form.instance
        for f in my_course.faculty.all():
            a, created = Assists.objects.get_or_create(user=self.request.user.userprofile, faculty=f)
        return super(CourseUpdate, self).form_valid(form)

Try to replace `my_course = form.instance` with `my_course = form.save(commit=False)` — Vingtoft, Oct 07 '15 at 17:36
@Vingtoft, thank you. When I try that, upon clicking the Save btn when creating a new course, I still see the same error:" The Course object needs to have a value for field 'course' before this many-to-many relationship can be used." I don't quite understand where the "course" mention comes from... I do have a class named Course, but it doesn't have a field named "course", and my view doesn't have any mention of a "course". — Carrie, Oct 07 '15 at 18:05

Alasdair · Accepted Answer · 2015-10-07T20:09:42.937

95

The form_valid() method for CreateView and UpdateView saves the form, then redirects to the success url. It's not possible to do return super(), because you want to do stuff in between the object being saved and the redirect.

The first option is to not call super(), and duplicate the two lines in your view. The advantage of this is that it's very clear what is going on.

def form_valid(self, form):
    self.object = form.save()
    # do something with self.object
    # remember the import: from django.http import HttpResponseRedirect
    return HttpResponseRedirect(self.get_success_url())

The second option is to continue to call super(), but don't return the response until after you have updated the relationship. The advantage of this is that you are not duplicating the code in super(), but the disadvantage is that it's not as clear what's going on, unless you are familiar with what super() does.

def form_valid(self, form):
    response = super(CourseCreate, self).form_valid(form)
    # do something with self.object
    return response

edited Oct 07 '15 at 20:09

answered Oct 07 '15 at 19:39

Alasdair

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Wow, thank you, this is working (I went with the first option for transparency). I did spend a lot of time looking through the documentation, but still find it hard to troubleshoot things like this myself. Thank you very much @Alasdair! – Carrie Oct 07 '15 at 20:07
2

Personally I prefer the first option as well. Glad it helped :-) – Alasdair Oct 07 '15 at 20:08
@Alasdair: How about overriding `form_valid()` and updating `form.instance` in it which is generated using POST data as result of [`form.is_valid`](https://github.com/django/django/blob/stable/1.11.x/django/views/generic/edit.py#L182) called just before `form_valid()`. This `instance` is later used to set `object` when `form.save()` is called in `form_valid` – Rahul Verma Jun 06 '18 at 11:44
@batMan if I remember correctly, `form.instance` is `None` for `CreateView`. You can set `instance` to `Course()` in `get_form_kwargs`, but that doesn't help you much for this particular question, because the instance has to be saved before you can set the values for the many-to-many field. – Alasdair Jun 06 '18 at 13:01

score 3 · Answer 2 · answered Feb 06 '17 at 21:47

3

I would suggest to use Django's Signal. That is an action that gets triggered when something happens to a Model, like save or update. This way your code stays clean (no business logic in the form-handling), and you are sure that it only gets triggered after save.

#views.py
from django.dispatch import receiver
...

@receiver(post_save, sender=Course)
def post_save_course_dosomething(sender,instance, **kwargs):
    the_faculty = instance.faculty
    #...etc

answered Feb 06 '17 at 21:47

Alex

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This won't work in this case because you cannot access the M2M fields like `faculty` using the `post_save` signal, see this answer https://stackoverflow.com/a/23796604/263989 – fasouto Apr 24 '18 at 22:25

score 2 · Answer 3 · answered May 26 '22 at 14:08

If you need to modify also the Course object when call save function use False and after change save the object

def form_valid(self, form):
    self.object = form.save(False)
    # make change at the object
    self.object.save()

    return HttpResponseRedirect(self.get_success_url())

score 0 · Answer 4 · answered Jan 05 '22 at 21:14

It is possible to do return super() as it is in the django doc: https://docs.djangoproject.com/en/4.0/topics/class-based-views/generic-editing/

def form_valid(self, form):
    # This method is called when valid form data has been POSTed.
    # It should return an HttpResponse.
    form.send_email()
    return super().form_valid(form)

Django CreateView: How to perform action upon save

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