Models.ModelsForm doesn't work, form.is_valid return false, Models.Form works fine

Using Django
1

Problem Description:
I got an invalid Form (form.is_valid() == False) using models.ModelForm class while models.Form class worked. I took several hours trying to figure it out, still don’t get the answer.

The following code works fine.

class UserInfo(models.Model):
    user_name = models.CharField(max_length=50, unique=True)
    password = models.CharField(max_length=256)
    email = models.EmailField(max_length=50, unique=True, blank=True)
    time_creation = models.DateTimeField(auto_now_add=True, null=True)

 def login(request):
     if request.method == 'POST':
         form = LoginForm(request.POST)
         if form.is_valid():
             username = form.cleaned_data.get('user_name')
             password = form.cleaned_data.get('password')
             try:
                  user = UserInfo.objects.get(user_name=username)
             except :
                 message = 'User does not exsit'
                 return render(request, 'manager/login_.html', {'form': form, 'message': message})
            if user.password == password:
            return redirect("manager/home.html")
            else:
                   message = 'Password is wrong'
                   return render(request, 'manager/login_.html', {'form': form, 'message': message})
         else:
               print('form is invalid')
               form = LoginForm()
               return render(request, 'manager/login.html', {'form': form})
     else:
          form = LoginForm()
          return render(request, 'manager/login_.html', {'form': form})

class LoginForm(forms.Form):
    user_name = forms.CharField(
                           max_length=50,
                           label='User',
    )
    password = forms.CharField(
                          max_length=50,
                          label='Password',
    )

If I change the form using Models.Form as follows, it won’t work:

class LoginForm(forms.ModelForm):
    class Meta:
        model = UserInfo
        fields = ('user_name', 'password')
        labels = {
             'user_name': "User",
             'password': "Password",
         }

When I enter an invalid username that doesn’t exist in the database. It prompts ‘User does not exsit’, which shows that it caught the exception. But when I enter a correct username with a valid or invalid password it returns invalid form( I insert a print(‘Invalid form’) in the code)

3

It would help if you identified what Django thinks is wrong with the form.

(You could, for this purpose, simply print the form errors)

5

Hi, I got the following message. I still don’t get what the problem is.

<ul class="errorlist"><li>user_name<ul class="errorlist"><li>用户 with this User name already exists.</li></ul></li></ul>
invalid
6

<conjecture>
I’m going to go out on a limb here and say that, in general terms, a ModelForm is designed to enter / edit data for a model. It appears as if the is_valid method is checking the form to see whether or not the data in the submitted form can be added to the database, and since that username already exists the answer is “no”.

I would say from what I’m seeing, a ModelForm isn’t the right tool for a login form - when the data being entered isn’t intended to insert or edit model data.
</conjecture>