SQLAlchemy query not retrieving committed data in alternate session - django

Question

I have a problem where I insert a database item using a SQLAlchemy / Tastypie REST interface, but the item is missing when subsequently get the list of items. It shows up only after I get the list of items a second time.
I am using SQLAlchemy with Tastypie/Django running on Apache via mod_wsgi. I use a singleton Database Manager class to hold my engine and declarative_base, and with Tastypie, a separate class to get the session and make sure I roll-back if there is a problem with the commit. As in the update below, the problem occurs when I don't close my session after inserting. Why is this necessary?
My original code was like this:
Session = scoped_session(sessionmaker(autoflush=True))
# Singleton Database Manager class for managing session
class DatabaseManager():
engine = None
base = None
def ready(self):
host='mysql+mysqldb://etc...'
if self.engine and self.base:
return True
else:
try:
self.engine = create_engine(host, pool_recycle=3600)
self.base = declarative_base(bind=self.engine)
return True
except:
return False
def getSession(self):
if self.ready():
session = Session()
session.configure(bind=self.engine)
return session
else:
return None
DM = DatabaseManager()
# A session class I use with Tastypie to ensure the session is destroyed at the
# end of the transaction, because Tastypie creates singleton Resources used for
# all threads
class MySession:
def __init__(self):
self.s = DM.getSession()
def safeCommit(self):
try:
self.s.commit()
except:
self.s.rollback()
raise
def __del__(self):
try:
self.s.commit()
except:
self.s.rollback()
raise
# ... Then ... when I get requests through Apache/mod_wsgi/Django/Tastypie
# First Request
obj_create():
db = MySession()
print db.s.query(DBClass).count() # returns 4
newItem = DBClass()
db.s.add(newItem)
db.s.safeCommit()
print db.s.query(DBClass).count() # returns 5
# Second Request after First Request returns
obj_get_list():
db = MySession()
print db.s.query(DBClass).count() # returns 4 ... should be 5
# Third Request is okay
obj_get_list():
db = MySession()
print db.s.query(DBClass).count() # returns 5
UPDATE
After further digging, it appears that the problem is my session needed to be closed after creating. Perhaps because Tastypie's object_create() adds the SQLAlchemy object to it's bundle, and I don't know what happens after it leaves the function's scope:
obj_create():
db = MySession()
newItem = DBClass()
db.s.add(newItem)
db.s.safeCommit()
copiedObj = copyObj(newItem) # copy SQLAlchemy record into non-sa object (see below)
db.s.close()
return copiedObj
If someone cares to explain this in an answer, I can close the question. Also, for those who are curious, I copy my object out of SQLAlchemy like this:
class Struct:
def __init__(self, **entries):
self.__dict__.update(entries)
class MyTastypieResource(Resource):
...
def copyObject(self, object):
base = {}
# self._meta is part of my tastypie resource
for p in class_mapper(self._meta.object_class).iterate_properties:
if p.key not in base and p.key not in self._meta.excludes:
base[p.key] = getattr(object,p.key)
return Struct(**base)

Answer 1

The problem was resolved by closing my session. The update in the answer didn't solve the problem fully - I ended up adding a middleware class to close the session at the end of a transaction. This ensured everything was written to the database. The middleware looks a bit like this:
class SQLAlchemySessionMiddleWare(object):
def process_response(self, request, response):
try:
session = MyDatabaseManger.getSession()
session.commit()
session.close()
except Exception, err:
pass
return response
def process_exception(self, request, exception):
try:
session = MyDatabaseManger.getSession()
session.rollback()
session.close()
except Exception, err:
pass