Django多对多关系表单报错：TypeError: Field 'id'期望数字却得到<Profile: user1>

问题描述 #

开发一款推荐应用，用户可向多位用户推荐影视、书籍。表单显示所有用户列表，勾选目标用户提交推荐时，出现错误：Field 'id' expected a number but got <Profile: user1>。尝试修改表单ModelMultipleChoiceField的queryset为values_list，仅返回类似(1,1)的元组，无法显示用户名也无法保存数据。

models.py #

from django.db import models
from django.contrib.auth.models import User
from django.db.models.signals import post_save
from django.dispatch import receiver

class Profile(models.Model):
    user = models.OneToOneField(User, on_delete=models.CASCADE)
    follows = models.ManyToManyField( 
        'self',
        related_name='followed_by',
        symmetrical=False,
        blank=True,
    )

    def __str__(self):
        return self.user.username
    
class Recommendation(models.Model):
    user = models.ForeignKey(
        User, related_name="recs", on_delete=models.DO_NOTHING
    )
    recipients = models.ManyToManyField(
        User, related_name="received_recs", symmetrical=False, blank=True
    )
    title = models.CharField(max_length=300)
    description = models.TextField(blank=True)
    created_at = models.DateTimeField(auto_now_add=True)

    def __str__(self):
        return (
            f"{self.user} "
            f"({self.created_at:%y-%m-%d %H:%M}): "
            f"{self.title}"
        )

views.py #

from django.shortcuts import render, redirect
from .models import Profile, Recommendation
from .forms import RecForm

def dashboard(request):
  all_recs = Recommendation.objects.all()
  if request.method == "POST":
    form = RecForm(request.POST or None)
    if form.is_valid():
      rec = form.save(commit=False)
      rec.user = request.user
      rec.save()
      form.save_m2m()
      return redirect("watchthisshit:dashboard")
  form = RecForm()
  return render(request, "watchthisshit/dashboard.html", {
    "all_recs": all_recs,
    "form": form
  })

forms.py #

from django import forms
from django.db.models.base import Model
from .models import Profile, Recommendation

class RecForm(forms.ModelForm):
  title = forms.CharField(required=True)
  description = forms.CharField(widget=forms.widgets.Textarea(
    attrs={
      "placeholder": "You don't have to write a description, but it would be pretty cool if you did...",
      "class": "textarea is-success is-medium"
    }
  ), label="",)
  recipients = forms.ModelMultipleChoiceField(
    queryset=Profile.objects.all(),
    widget=forms.CheckboxSelectMultiple
  )

  class Meta:
    model = Recommendation
    fields = ["title", "description", "recipients"]
    exclude = ("user", )

错误原因 #

表单的recipients字段关联了Profile模型，但Recommendation模型的recipients字段实际关联的是User模型。提交表单时，表单传递的是Profile对象，而数据库期望接收User的ID，导致类型不匹配，触发报错。

之前尝试用values_list的方式无效，因为ModelMultipleChoiceField需要接收模型实例的queryset，而非元组数据，无法正确识别和保存对象。

解决步骤 #

方案一：调整表单关联User模型（推荐） #

修改forms.py中的recipients字段，将queryset改为User.objects.all()，同时可自定义显示文本：

class RecForm(forms.ModelForm):
  title = forms.CharField(required=True)
  description = forms.CharField(widget=forms.widgets.Textarea(
    attrs={
      "placeholder": "You don't have to write a description, but it would be pretty cool if you did...",
      "class": "textarea is-success is-medium"
    }
  ), label="",)
  recipients = forms.ModelMultipleChoiceField(
    queryset=User.objects.all(),
    widget=forms.CheckboxSelectMultiple,
    # 自定义显示用户名，若需显示Profile中字段可调整逻辑
    label_from_instance=lambda obj: obj.username
  )

  class Meta:
    model = Recommendation
    fields = ["title", "description", "recipients"]
    exclude = ("user", )

方案二：修改模型关联Profile（若业务需要） #

如果业务逻辑更适合关联Profile而非User，修改models.py的Recommendation模型：

class Recommendation(models.Model):
    user = models.ForeignKey(
        User, related_name="recs", on_delete=models.DO_NOTHING
    )
    # 将关联对象改为Profile
    recipients = models.ManyToManyField(
        Profile, related_name="received_recs", symmetrical=False, blank=True
    )
    title = models.CharField(max_length=300)
    description = models.TextField(blank=True)
    created_at = models.DateTimeField(auto_now_add=True)

    def __str__(self):
        return (
            f"{self.user} "
            f"({self.created_at:%y-%m-%d %H:%M}): "
            f"{self.title}"
        )

此方案下，原表单的Profile queryset与模型匹配，无需修改表单，且Profile的__str__方法已返回用户名，显示正常。

额外提示 #

• 若使用方案一，如需显示Profile中的自定义字段（如昵称），可调整label_from_instance逻辑：lambda obj: obj.profile.nickname if hasattr(obj, 'profile') else obj.username
• 执行模型修改后，需运行python manage.py makemigrations和python manage.py migrate更新数据库。

内容的提问来源于stack exchange，提问作者Sarah