TypeError
TypeError: ObjectId('51e1cfda4ed6db61932a0a0a') is not JSON serializable

Traceback (most recent call last)
File "/home/alex/dev/repos/minitwit-mongodb/env/lib/python2.7/site-packages/flask/app.py", line 1836, in __call__
return self.wsgi_app(environ, start_response)
File "/home/alex/dev/repos/minitwit-mongodb/env/lib/python2.7/site-packages/flask/app.py", line 1820, in wsgi_app
response = self.make_response(self.handle_exception(e))
File "/home/alex/dev/repos/minitwit-mongodb/env/lib/python2.7/site-packages/flask/app.py", line 1403, in handle_exception
reraise(exc_type, exc_value, tb)
File "/home/alex/dev/repos/minitwit-mongodb/env/lib/python2.7/site-packages/flask/app.py", line 1817, in wsgi_app
response = self.full_dispatch_request()
File "/home/alex/dev/repos/minitwit-mongodb/env/lib/python2.7/site-packages/flask/app.py", line 1479, in full_dispatch_request
response = self.process_response(response)
File "/home/alex/dev/repos/minitwit-mongodb/env/lib/python2.7/site-packages/flask/app.py", line 1693, in process_response
self.save_session(ctx.session, response)
File "/home/alex/dev/repos/minitwit-mongodb/env/lib/python2.7/site-packages/flask/app.py", line 837, in save_session
return self.session_interface.save_session(self, session, response)
File "/home/alex/dev/repos/minitwit-mongodb/env/lib/python2.7/site-packages/flask/sessions.py", line 326, in save_session
val = self.get_signing_serializer(app).dumps(dict(session))
File "/home/alex/dev/repos/minitwit-mongodb/env/lib/python2.7/site-packages/itsdangerous.py", line 537, in dumps
payload = want_bytes(self.dump_payload(obj))
File "/home/alex/dev/repos/minitwit-mongodb/env/lib/python2.7/site-packages/itsdangerous.py", line 809, in dump_payload
json = super(URLSafeSerializerMixin, self).dump_payload(obj)
File "/home/alex/dev/repos/minitwit-mongodb/env/lib/python2.7/site-packages/itsdangerous.py", line 522, in dump_payload
return want_bytes(self.serializer.dumps(obj))
File "/home/alex/dev/repos/minitwit-mongodb/env/lib/python2.7/site-packages/flask/sessions.py", line 85, in dumps
return json.dumps(_tag(value), separators=(',', ':'))
File "/home/alex/dev/repos/minitwit-mongodb/env/lib/python2.7/site-packages/flask/json.py", line 126, in dumps
rv = _json.dumps(obj, **kwargs)
File "/usr/lib/python2.7/json/__init__.py", line 238, in dumps
**kw).encode(obj)
File "/usr/lib/python2.7/json/encoder.py", line 203, in encode
chunks = list(chunks)
File "/usr/lib/python2.7/json/encoder.py", line 428, in _iterencode
for chunk in _iterencode_dict(o, _current_indent_level):
File "/usr/lib/python2.7/json/encoder.py", line 402, in _iterencode_dict
for chunk in chunks:
File "/usr/lib/python2.7/json/encoder.py", line 436, in _iterencode
o = _default(o)
File "/home/alex/dev/repos/minitwit-mongodb/env/lib/python2.7/site-packages/flask/json.py", line 83, in default
return _json.JSONEncoder.default(self, o)
File "/usr/lib/python2.7/json/encoder.py", line 178, in default
raise TypeError(repr(o) + " is not JSON serializable")
TypeError: ObjectId('51e1cfda4ed6db61932a0a0a') is not JSON serializable
The debugger caught an exception in your WSGI application. You can now look at the traceback which led to the error.
To switch between the interactive traceback and the plaintext one, you can click on the "Traceback" headline. From the text traceback you can also create a paste of it. For code execution mouse-over the frame you want to debug and click on the console icon on the right side.

You can execute arbitrary Python code in the stack frames and there are some extra helpers available for introspection:

dump() shows all variables in the frame
dump(obj) dumps all that's known about the object