更新数据库中已存在的数据

Question

我正在尝试使用“update（）”来更新“ModelC”数据库中的现有数据。

型号：

from django.db import models

class ModelA(models.Model):
     first_name = models.CharField(max_length=20)
     last_name = models.CharField(max_length=20)

     def __str__(self):
         return '%s' %(self.id)

class ModelB(models.Model):
     observation = models.CharField(max_length=30)
     status = models.BooleanField()
     relation = OneToOneField(ModelA)

     def __str__(self):
         return '%s' %(self.relation) 

class ModelC(models.Model):
     relationA = models.ForeignKey('ModelA', null=True, blank=True)
     date_relationA = models.CharField(max_length=10, null=True, blank=True)
     user_relationA = models.CharField(max_length=15, null=True, blank=True)
     relationB = models.ForeignKey('ModelB', null=True, blank=True)
     date_relationB = models.CharField(max_length=10, null=True, blank=True)
     user_relationB = models.CharField(max_length=15, null=True, blank=True)

     def __str__(self):
         return str(self.id)

ModelA的观点：

def RegModelA(request):
   form = ""
   user = None
   if request.method == "POST":
      form = ModelAForm(request.POST)
      if form.is_valid():
         save = form.save()
         date = time.strftime("%d - %m - %Y")
         user = request.user.username
         create = ModelC.objects.create(relationA=save, date_relationA=date, user_relationA=user, relationB=None, date_relationB=None, user_relationB=None)
         return redirect('/')
   else:
      form = ModelAForm
   return render(request, "ModelA.html", {"form":form})

**注册ModelA时的结果：**

在ModelA中：

Image of the result

在ModelC中：

Image of the result

ModelB的观点：

def RegModelB(request):
   form = ""
   user = None
   if request.method == "POST":
      form = ModelBForm(request.POST)
      if form.is_valid():
         save = form.save()
         date = time.strftime("%d - %m - %Y")
         user = request.user.username
         update = ModelC.objects.filter().update(relationB=save, date_relationB=date, user_relationB=user)
         return redirect('/')
   else:
      form = ModelBForm
   return render(request, "ModelB.html", {"form":form})

我在寻找的是，当注册ModelB的数据时，ModelC中关系B的字段会被更新。

但是如果ModelB的关系等于ModelC中的字段relationA，我就找不到如何获取ModelC的ID。

我正在使用：

• Python 3.5

• Django 1.11