[FIXED] Set specific validation rule in UpdateView

Issue

I set an UpdateView class to update data of user's markers. I have a Marker model with a field category. I want to have one and only "Where I live" category marker in all the user's markers. Is there a way to create a function in the UpdateView class to invalidate the form if there are more than one "Where I live" markers.

class UpdateMarkerView(UpdateView):
    model = Marker
    fields = ["name", "description", "category"]
    template_name = "map/detail.html"
    success_url = "/"

class Marker(models.Model):
    WIL = "WIL"
    WIWG = "WIWG"
    WIW = "WIW"
    IDK = "IDK"
    
    CATEGORY_MARKER = [(WIL,"Where I live"), (WIWG, "Where I want to go"),(WIW,"Where I went"),(IDK,"I don't know")]
    user = models.ForeignKey(User,blank=True, null=True,on_delete = models.SET_NULL)
    name = models.fields.CharField(max_length=100)
    description = models.fields.CharField(max_length=100)
    lat = models.fields.DecimalField(default=False, max_digits=5, decimal_places=3)
    lon = models.fields.DecimalField(default=False, max_digits=5, decimal_places=3)
    category = models.CharField(max_length=50, choices = CATEGORY_MARKER, default = IDK)

I've already done it with a basic function but I can't find a way to do it with a generic view if it's possible.

Solution

You can use the form_valid method for the general answer.

But if you want to return an invalid form when there was more than 1 "Where I Live" category, you can write it like this:

class UpdateMarkerView(UpdateView):
    model = Marker
    fields = ["name", "description", "category"]
    template_name = "map/detail.html"
    success_url = "/"

    def form_valid(self, form):
        data = form.cleaned_data
        if data.get("category") == "WIL":
           if Marker.objects.filter(category="WIL").count() >= 1:
              form.add_error('category', 'This category already exists.')
              return self.form_invalid(form)
        else:
            return super().form_valid(form)

First we check that the category selected by the user is "WIL", then if there is more than 1 category(WIL), we add an error to the form and return the invalid form.

You can also change count to the exists in the query, because it means that one already exists and this is the second.

Answered By - zankoAN