Python Django 设置默认表单值

我有一个模型如下:

class TankJournal(models.Model):
    user = models.ForeignKey(User)
    tank = models.ForeignKey(TankProfile)
    ts = models.IntegerField(max_length=15)
    title = models.CharField(max_length=50)
    body = models.TextField()

我还有一份上述模型的模型表格，如下所示:

class JournalForm(ModelForm):
    tank = forms.IntegerField(widget=forms.HiddenInput()) 

    class Meta:
        model = TankJournal
        exclude = ('user','ts')

我想知道如何设置坦克隐藏字段的默认值.以下是我迄今为止显示/保存表单的功能:

def addJournal(request, id=0):
    if not request.user.is_authenticated():
        return HttpResponseRedirect('/')

    # checking if they own the tank
    from django.contrib.auth.models import User
    user = User.objects.get(pk=request.session['id'])

    if request.method == 'POST':
        form = JournalForm(request.POST)
        if form.is_valid():
            obj = form.save(commit=False)

            # setting the user and ts
            from time import time
            obj.ts = int(time())
            obj.user = user

            obj.tank = TankProfile.objects.get(pk=form.cleaned_data['tank_id'])

            # saving the test
            obj.save()

    else:
        form = JournalForm()

    try:
        tank = TankProfile.objects.get(user=user, id=id)
    except TankProfile.DoesNotExist:
        return HttpResponseRedirect('/error/')