Generate a unique username for django.contrib.auth

[Micah Carrick]

I've got my site's authentication working with and email and password only--no username (thanks to Shawn Milochik for helping me with that). However, I still need to put in a username to make the User model happy. I was hoping to have "user" as a prefix and then some unique number.

I cannot simply copy the email to the username because the username must be less than 30 characters and, after looking into my database, many email addresses go over that.
I cannot generate a random number because there could be a collision.
I cannot use uuid4().hex because that's 32 characters... I need <30.
I cannot use User.objects.count() because that could result in a collision if 2 users register at the same time.

Thoughts?

[marco carminati]

Hi Micah

On Jan 12, 10:11 pm, Micah Carrick <mi...@greentackle.com> wrote:
> I've got my site's authentication working with and email and password
> only--no username (thanks to Shawn Milochik for helping me with that).
> However, I still need to put in a username to make the User model happy. I
> was hoping to have "user" as a prefix and then some unique number.
>

couldn't you use the user.id after creating it?
i.e.

prefix = 'user'
###some code to save user###

user.your_field = prefix + str(user.id)
user.save()

if you don't neet this value in the database you could put in your
mail directly the user.id
<h1>hello {{ user.id }}</h1>

unfortunately I cannot test this solution now (sorry, i've not django
in this machine)

hoping this help,
marco

[Eric Chamberlain]

We use a base64 or base36 (if you want compatibility with the contrib.admin) encoded UUID, or generate a random 30-character string, the odds of a collision is quite low.

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[Acorn]

Why not just use incremental numeric user IDs?

[Micah Carrick]

Thanks folks.

Here's what I have now. Not the most elegant but it's the best I can come up with so far. First, I'm generating a random string for the username so that the form will validate. After the record is saved I change the username to "user_" and the user ID. This way it looks a little less scary if it pops up on an admin form somewhere or something.

# in the view for my sign up form
if request.method == 'POST':
        data = request.POST.copy()
        randstr = ''.join([choice(string.letters) for i in xrange(30)])
        data['username'] = 'user_' + randstr
        form = SignUpForm(data) # subclass of UserCreationForm
        if form.is_valid():
            form.save()

# in the save method of SignUpForm which is subclassed from UserCreationForm
user.save()
user.username = 'user_' + str(user.id)
user.save()

What I still hate about this one is that I'm hitting the database twice for a single registration. But, it seems to work.

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[Eric Chamberlain]

Because the id is not known until after the record is saved, so you'd have to generate some non-colliding filler value anyway. Using incremental numbers can also leak usage information to the users.

[Ian Clelland]

On Wed, Jan 12, 2011 at 1:11 PM, Micah Carrick <mi...@greentackle.com> wrote:
> I cannot use uuid4().hex because that's 32 characters... I need <30.

uuid4().hex[:30] is almost as random as uuid4().hex

There are no timers, MAC addresses, or other non-random sections in a
UUID4, so taking any 30-character slice from it should get you 120
bits of pure random goodness.

--
Regards,
Ian Clelland
<clel...@gmail.com>

[Martin J. Laubach]

> I cannot simply copy the email to the username because the username must be
> less than 30 characters and, after looking into my database, many email
> addresses go over that.

Note that you can fix that quite easily by putting something like
this

for f in User._meta.fields:
if f.name == "username":
f.max_length=75

somewhere in your code. Easiest in your models.py.

mjl