Vineet Kaushik Vineet Kaushik - 25 days ago 7
Python Question

Unable to check for already existing usernames google app engine

Here is my code to check for already existing usernames :

def post(self):
have_error = False
username = self.request.get('username')
password = self.request.get('password')
verify = self.request.get('verify')
email = self.request.get('email')

params = dict(username= username,
email = email)

usernames = db.GqlQuery(' select * from Users where username = :1 ', username)


if not valid_username(username):
params['username_error'] = "Invalid username"
have_error = True

if usernames :
params['username_error'] = 'Username already exists'
have_error = True


With this code my form displays
Username already exists
for every username.

When the
params['username_error'] = 'Username already exists'
is changed to
params['username_error'] = usernames
,

the message displayed in the
username_error
field is :

<google.appengine.ext.db.GqlQuery object at 0x10a4e030>

Answer

I'd change your check to:

query = db.GqlQuery(' select *  from Users where username = :1 ', username)
usernames = query.count(limit=2)

Now, the if usernames: guard will have the effect you desire.

The reason is simple: in your code, you build a query object, which you call usernames, but you never actually run that query -- you check the object for truthiness, which happens to hold (as does for almost all Python objects except empty containers and strings, None, zero numbers) but signifies nothing.

Here, I run the query in the simplest way -- just count how many items it would return (just-counting is a bit faster than actually fetching them:-) and with a very low limit (since we only care about counts of 0, which is correct, and 1, which means a duplicate).

Now usernames is a number counting how many other times that user name appears in the datastore (with 1 standing for "once or more":-) and the rest of your logic becomes accurate.