Add unique username check when creating users.

tailbone/views/users.py

@@ -27,7 +27,7 @@ User Views
 
 from __future__ import unicode_literals
 
-from formalchemy import Field
+from formalchemy import Field, ValidationError
 from formalchemy.fields import SelectFieldRenderer
 
 from edbob.pyramid.views import users
@@ -168,6 +168,12 @@ class UserCrud(CrudView):
                 fs.active,
                 ])
 
+        if self.creating:
+            def valid_username(value, field):
+                if Session.query(User).filter_by(username=value).count():
+                    raise ValidationError("Username must be unique.")
+            fs.username.set(validate=valid_username)
+
         if self.readonly:
             del fs.password
             del fs.confirm_password