Getting object representing the current user

I have the following model (stripped of comments and __unicode__ for
brevity) which defines a comment in my Django app (I'm not using the
comment module provided with Django for various reasons):

class Comment(models.Model):
comment = models.TextField()
added_by = models.ForeignKey(User)
added_date = models.DateTimeField(auto_now_add = True)
approved = models.BooleanField()

class CommentForm(ModelForm):
class Meta:
model = Comment
fields = ['comment']

I also have a simple template which just displays the CommentForm
(i.e. a textarea within a form element) and the action of the form is
set to the following view:

def add_comment(request):
if request.method == 'POST' and request.user.is_authenticated():
comment = Comment()
comment.added_by = request.user
comment.approved = False

comment_form = CommentForm(request.POST, instance = comment)

if comment_form.is_valid():
comment.save()
return HttpResponseRedirect('/')

However, when I submit the form I get the following error:

(1048, "Column 'added_by_id' cannot be null")

I'm sure request.user must be set, because it is being accessed
earlier in the code (request.user.is_authenticated), so I'm not sure
why this error is appearing. Am I doing something obviously wrong? I'm
new to both Django and Python so I am picking things up as I go along.

Thanks in advance for any pointers.

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