如何用 TextInput 替换 Django ModelForm 中的 Choices 字段

我正在使用 Python 3.7.3 和 Django 2.0.13。

基本上我想在我的网站上显示一个表单，可以在其中输入用户（=下面模型定义中的参与者）。Django ModelForm 自动将其设置为选择字段并显示所有用户的下拉列表。我不想在下拉菜单中显示所有用户的列表，而是想要一个 TextInput 字段。

代码：

首先，来自models.py的相关部分：

class Invite(models.Model):
    game        = models.ForeignKey(Game, on_delete=models.CASCADE)

    host        = models.ForeignKey(User, on_delete=models.CASCADE, related_name = "invites_as_host")
    participant = models.ForeignKey(User, on_delete=models.CASCADE, related_name = "invites_as_participant")

    accepted = models.BooleanField(blank = True, default = False)
    declined = models.BooleanField(blank = True, default = False)

    date_created = models.DateTimeField(auto_now_add=True)
    date_edited  = models.DateTimeField(auto_now=True)

    class Meta:
        unique_together = ["game", "host", "participant"]

表格.py：

class GameInviteNewForm(forms.ModelForm):
    class Meta:
        model = Invite
        fields = ["participant"]

我尝试的是像这样覆盖参与者输入字段：

class GameInviteNewForm(forms.ModelForm):

    participant = forms.CharField(
                label=_("User to invite"), 
                max_length=100,
                widget = forms.TextInput
            )

    class Meta:
        model = Invite
        fields = ["participant"]

views.py（如果相关；我认为它甚至不会达到“form_valid”，是吗？）

class GameInviteNewView(LoginRequiredMixin, UserIsLeaderMixin, FormView):
    form_class = GameInviteNewForm 

    template_name = "app/game/new_invite.html"

    pk_url_kwarg  = "game_id"


    def get_success_url(self):
        return reverse_lazy("app:game_invite", kwargs={
            "game_id": self.kwargs['game_id']
        })


    def form_valid(self, form):
        participant = form.save(commit=False)

        participant = User.objects.get(username=participant.name)
        host = User.objects.get(username=self.request.user.username)
        game = Game.objects.get(id=self.kwargs['game_id'])

        invite.participant_id = participant.id
        invite.host_id = host.id
        invite.game_id = game.id

        invite.save()

        return redirect(self.get_success_url())

这确实在网站上显示了一个 TextInput 字段，但是如果我输入用户名（“test”），我会收到一个错误：

Internal Server Error: /app/game/invite/7
Traceback (most recent call last):
  File "/home/dremet/anaconda3/envs/django/lib/python3.7/site-packages/django/core/handlers/exception.py", line 35, in inner
    response = get_response(request)
  File "/home/dremet/anaconda3/envs/django/lib/python3.7/site-packages/django/core/handlers/base.py", line 128, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "/home/dremet/anaconda3/envs/django/lib/python3.7/site-packages/django/core/handlers/base.py", line 126, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/home/dremet/anaconda3/envs/django/lib/python3.7/contextlib.py", line 74, in inner
    return func(*args, **kwds)
  File "/home/dremet/anaconda3/envs/django/lib/python3.7/site-packages/django/views/generic/base.py", line 69, in view
    return self.dispatch(request, *args, **kwargs)
  File "/home/dremet/anaconda3/envs/django/lib/python3.7/site-packages/django/contrib/auth/mixins.py", line 52, in dispatch
    return super().dispatch(request, *args, **kwargs)
  File "/home/dremet/anaconda3/envs/django/lib/python3.7/site-packages/django/contrib/auth/mixins.py", line 109, in dispatch
    return super().dispatch(request, *args, **kwargs)
  File "/home/dremet/anaconda3/envs/django/lib/python3.7/site-packages/django/views/generic/base.py", line 89, in dispatch
    return handler(request, *args, **kwargs)
  File "/home/dremet/anaconda3/envs/django/lib/python3.7/site-packages/django/views/generic/edit.py", line 141, in post
    if form.is_valid():
  File "/home/dremet/anaconda3/envs/django/lib/python3.7/site-packages/django/forms/forms.py", line 179, in is_valid
    return self.is_bound and not self.errors
  File "/home/dremet/anaconda3/envs/django/lib/python3.7/site-packages/django/forms/forms.py", line 174, in errors
    self.full_clean()
  File "/home/dremet/anaconda3/envs/django/lib/python3.7/site-packages/django/forms/forms.py", line 378, in full_clean
    self._post_clean()
  File "/home/dremet/anaconda3/envs/django/lib/python3.7/site-packages/django/forms/models.py", line 396, in _post_clean
    self.instance = construct_instance(self, self.instance, opts.fields, opts.exclude)
  File "/home/dremet/anaconda3/envs/django/lib/python3.7/site-packages/django/forms/models.py", line 60, in construct_instance
    f.save_form_data(instance, cleaned_data[f.name])
  File "/home/dremet/anaconda3/envs/django/lib/python3.7/site-packages/django/db/models/fields/__init__.py", line 838, in save_form_data
    setattr(instance, self.name, data)
  File "/home/dremet/anaconda3/envs/django/lib/python3.7/site-packages/django/db/models/fields/related_descriptors.py", line 197, in __set__
    self.field.remote_field.model._meta.object_name,
ValueError: Cannot assign "'test'": "Invite.participant" must be a "User" instance.

下拉列表在每个选项的“值”属性中都有用户 ID。现在，输入一个字符串。所以我对它不起作用并不感到惊讶，但我很惊讶错误消息说它必须是“用户”（而不是用户 ID）。我试图覆盖“clean()”方法并使用常规形式，但都没有成功。这应该如何正确处理？

解决方案：

正如答案中所指出的，我确实需要一个“clean_participant”方法，但是我在它周围包裹了一个 try-except 结构（并且我坚持使用 forms.py 更改覆盖参与者字段）：

    def clean_participant(self):

        participant_string = self.cleaned_data['participant']

        try:
            participant = User.objects.get(username=participant_string)
        except User.DoesNotExist:
            raise forms.ValidationError("User does not exist.")

        return participant