Save user input which is a string as object in db

Question

I am trying to implement a registration key in the registration form which the user has to input to be able to create an account. In my users model, I have a key field which is the foreign key of the Key model. The relation looks like this

class RegistrationKey(models.Model):
    key = models.CharField(max_length=30)


class User(AbstractUser):
  ...
    key = models.ForeignKey(RegistrationKey, on_delete=models.CASCADE, null=True, blank=True)

I have tried to validate the input like this in the view

def registerPage(request):
    form = MyUserCreationForm()

    if request.method == 'POST':
        form = MyUserCreationForm(request.POST)
        key_input = request.POST.get('key')
...
       
        if RegistrationKey.objects.filter(key_input=key).exists() and form.is_valid():
            user = form.save(commit=False)
            user.key.add('key_input')
...

           user.save()

In my form have I set the key field as a CharField as I can not display the keys for the user. This means that the user inputs a string but django needs an instance to be able to save it to the database. I don't know how to implement this. Is there any way to convert a string to an instance or is there a better way to query the the database, to check and see if the input is a valid key?

Edit

This is the error when I try to add in the key 'Admin' which is in the database. "Cannot assign "'Admin'": "User.key" must be a "RegistrationKey" instance."

Answer 1

def registerPage(request):
    form = MyUserCreationForm()
    if request.method == 'POST':
        form = MyUserCreationForm(request.POST)
    
        if form.is_valid():
            key_input = form.cleaned_data['key_input']
            registration_key_object = RegistrationKey.objects.get(      
            key=key_input)
            user.key = registration_key_object
            user.save()

...

if you have a string(your_key) and you need object(RegistrationKey) simply get it reigstration_key_object = RegistrationKey.objects.get(key=your_key)

Answer 2

There are multiple approach to implement this

Approach 1:

def registerPage(request):
    form = MyUserCreationForm()

    if request.method == 'POST':
        form = MyUserCreationForm(request.POST)
        key_input = request.POST.get('key')
        registration_key = RegistrationKey.objects.filter(key_input=key_input)
        if Rregistration_key.exists() and form.is_valid():
            registration_key = registration_key.first()
            user = form.save(commit=False)
            user.key = registration_key
            user.save()

Approach 2:

def registerPage(request):
    form = MyUserCreationForm()

    if request.method == 'POST':
        form = MyUserCreationForm(request.POST)
        key_input = request.POST.get('key')
        if form.is_valid():
            try:
                registration_key = RegistrationKey.objects.get(key_input=key_input)
            except RegistrationKey.DoesNotExist:
                pass
                # raise your exception here
            user = form.save(commit=False)
            user.key = registration_key
            user.save()