How can get the valid form?

Question

When adding a quote which must reffer to author id I get the bellow error: <QueryDict: {'csrfmiddlewaretoken': ['qcbbM4KsxyH5jIOZYUEY4sUzy63TylU1oBgVn8bNX9EAHDcnGvoO1k4Oq0Yy5f3C'], 'quote': ['gf'], 'author': ['']}>

Form is not valid:

author

This field is required.

By default the database connection is set to postgres, however for the quotes and authors I'm using MongoDB.

I would appreciate it if someone could point out my mistakes.

forms.py

class QuoteForm(ModelForm):
    quote = CharField(max_length=50, widget=TextInput(attrs={"class": "form-control", "id": "exampleInputQuote1"}))
    #tags = CharField(max_length=50, widget=TextInput(attrs={"class": "form-control", "id": "exampleInputQuote2"}))
    author = ModelChoiceField(queryset=Author.objects.all(),
                              widget=Select(attrs={"class": "form-control", "id": "exampleInputQuote3"}))

models.py

class Quote(models.Model):
    quote = models.TextField()
    tags = models.ManyToManyField(Tag, blank=True, null=True)
    author = models.ForeignKey(Author, on_delete=models.CASCADE, default=None, null=True)
    created_at = models.DateTimeField(auto_now_add=True)

add_quote.html

<div class="mb-3">
            <label for="exampleInputQuote3" class="form-label">Author</label>
            <select name="author" class="form-control" id="exampleInputQuote3" required="">
                {% for author in authors %}
                <option value="{{ author.pk }}">{{ author.fullname }}</option>
                {% endfor %}
            </select>
            <div id="textHelp3" class="form-text">Quote Author.</div>
            <span></span>
        </div>

views.py

def add_quote(request):
    db = get_mongodb()
    authors = db.authors.find()
    form = QuoteForm(instance=Quote())

    if request.method == "POST":
        form = QuoteForm(request.POST, instance=Quote())
        print(request.POST)
        if form.is_valid():
            quote_text = form.cleaned_data['quote']
            author_id = form.cleaned_data['author'].id


            db.quotes.insert_one({"quote": quote_text, "author_id": author_id})
            return redirect(to="quotes:root")
        else:
            print("Form is not valid:", form.errors)

    return render(request, "quotes/add_quote.html", context={"form": form, "authors": authors})