SQLAlchemy query sometimes throws StatementError

I have a Flask application that uses SQLAlchemy connected to a mysql db running on my production server (Ubuntu machine).

75% of the time, when I issue a specific POST request, the application raises a StatementError with the following glass.

  [2015-06-18 17:25:46,512] {/home/dsaltares/conjugate/env/local/lib/python2.7/site-packages/flask/app.py:1423} ERROR - Exception on /conjugate/conjugate [POST]
Traceback (most recent call last):
  File "/home/dsaltares/conjugate/env/local/lib/python2.7/site-packages/flask/app.py", line 1817, in wsgi_app
    response = self.full_dispatch_request()
  File "/home/dsaltares/conjugate/env/local/lib/python2.7/site-packages/flask/app.py", line 1477, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/home/dsaltares/conjugate/env/local/lib/python2.7/site-packages/flask/app.py", line 1381, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/home/dsaltares/conjugate/env/local/lib/python2.7/site-packages/flask/app.py", line 1475, in full_dispatch_request
    rv = self.dispatch_request()
  File "/home/dsaltares/conjugate/env/local/lib/python2.7/site-packages/flask/app.py", line 1461, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "./conjugate.py", line 169, in conjugate
    for translation in get_translations(lang, verb)],
  File "./conjugate.py", line 80, in get_translations
    for entry in session.query(Translation).filter_by(lang=lang, english=english):
  File "/home/dsaltares/conjugate/env/local/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 2515, in __iter__
    return self._execute_and_instances(context)
  File "/home/dsaltares/conjugate/env/local/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 2530, in _execute_and_instances
    result = conn.execute(querycontext.statement, self._params)
  File "/home/dsaltares/conjugate/env/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 914, in execute
    return meth(self, multiparams, params)
  File "/home/dsaltares/conjugate/env/local/lib/python2.7/site-packages/sqlalchemy/sql/elements.py", line 323, in _execute_on_connection
    return connection._execute_clauseelement(self, multiparams, params)
  File "/home/dsaltares/conjugate/env/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1010, in _execute_clauseelement
    compiled_sql, distilled_params
  File "/home/dsaltares/conjugate/env/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1078, in _execute_context
    None, None)
  File "/home/dsaltares/conjugate/env/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1339, in _handle_dbapi_exception
    exc_info
  File "/home/dsaltares/conjugate/env/local/lib/python2.7/site-packages/sqlalchemy/util/compat.py", line 199, in raise_from_cause
    reraise(type(exception), exception, tb=exc_tb)
  File "/home/dsaltares/conjugate/env/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1071, in _execute_context
    conn = self._revalidate_connection()
  File "/home/dsaltares/conjugate/env/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 391, in _revalidate_connection
    "Can't reconnect until invalid "
StatementError: (sqlalchemy.exc.InvalidRequestError) Can't reconnect until invalid transaction is rolled back [SQL: u'SELECT translations.lang AS translations_lang, translat    ions.verb AS translations_verb, translations.english AS translations_english, translations.description AS translations_description \nFROM translations \nWHERE translations.l    ang = %s AND translations.english = %s'] [parameters: [immutabledict({})]]

      

        
        
        
      

    

The other 25% of the queries work very well and it returns the expected data. These requests are spaced out in time, it does not seem like someone is cheating the service.

The request fails in is

try:
    for entry in session.query(Translation).filter_by(lang=lang, english=english):
        translations.append({
            'lang': lang,
            'english': english,
            'verb': entry.verb,
            'description': entry.description
        })
except:
    app.logger.error('Error querying translations for (%s, %s)\n%s' % (lang, english, traceback.format_exc()))
    raise

      

        
        
        
      

    

Translation converter:

from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer, String

Base = declarative_base()

class Translation(Base):
    __tablename__ = 'translations'

    lang = Column(String(10), primary_key=True)
    verb = Column(String(25), primary_key=True)
    english = Column(String(25), primary_key=True)
    description = Column(String)

    def __repr__(self):
        return "<Translation(lang='%s', verb='%s', english='%s'>" % (self.lang, self.verb, self.english)

      

        
        
        
      

    

Any ideas as to what this might be? Do I need to provide additional information?