500 error when user changes their username to an already-taken one

[toolness]

I suspect that this is because the Discourse API is responding with an error due to the fact that the username is taken.

We should probably verify on the server-side (in NoI) that the username isn't already taken.

[toolness]

Oh, I was wrong about this being a discourse API error though, looks like the error is actually coming from NoI:

Exception on /me [POST]
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/site-packages/flask/app.py", line 1817, in wsgi_app
    response = self.full_dispatch_request()
  File "/usr/local/lib/python2.7/site-packages/flask/app.py", line 1477, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/usr/local/lib/python2.7/site-packages/flask/app.py", line 1381, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/local/lib/python2.7/site-packages/flask/app.py", line 1475, in full_dispatch_request
    rv = self.dispatch_request()
  File "/usr/local/lib/python2.7/site-packages/flask/app.py", line 1461, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/usr/local/lib/python2.7/site-packages/flask_login.py", line 758, in decorated_view
    return func(*args, **kwargs)
  File "/noi/app/views.py", line 77, in decorated_view
    return func(*args, **kwargs)
  File "/noi/app/views.py", line 181, in my_profile
    db.session.commit()
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/scoping.py", line 150, in do
    return getattr(self.registry(), name)(*args, **kwargs)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 790, in commit
    self.transaction.commit()
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 392, in commit
    self._prepare_impl()
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 372, in _prepare_impl
    self.session.flush()
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 2004, in flush
    self._flush(objects)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 2122, in _flush
    transaction.rollback(_capture_exception=True)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/util/langhelpers.py", line 60, in __exit__
    compat.reraise(exc_type, exc_value, exc_tb)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 2086, in _flush
    flush_context.execute()
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/unitofwork.py", line 373, in execute
    rec.execute(self)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/unitofwork.py", line 532, in execute
    uow
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/persistence.py", line 170, in save_obj
    mapper, table, update)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/persistence.py", line 692, in _emit_update_statements
    execute(statement, multiparams)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 914, in execute
    return meth(self, multiparams, params)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/sql/elements.py", line 323, in _execute_on_connection
    return connection._execute_clauseelement(self, multiparams, params)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1010, in _execute_clauseelement
    compiled_sql, distilled_params
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1146, in _execute_context
    context)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1341, in _handle_dbapi_exception
    exc_info
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/util/compat.py", line 199, in raise_from_cause
    reraise(type(exception), exception, tb=exc_tb)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1139, in _execute_context
    context)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/engine/default.py", line 450, in do_execute
    cursor.execute(statement, parameters)
IntegrityError: (psycopg2.IntegrityError) duplicate key value violates unique constraint "ix_users_username"
DETAIL:  Key (username)=(boop) already exists.
 [SQL: 'UPDATE users SET username=%(username)s, updated_at=%(updated_at)s WHERE users.id = %(users_id)s'] [parameters: {'username': u'boop', 'users_id': 216, 'updated_at': datetime.datetime(2016, 5, 26, 17, 18, 35, 745280)}]