Hello everyone...
I am getting detached instances error and I don't really know why.
There's something I don't get with the session, but I have been
reading the documentation thoroughly, trying many things and I can't
get rid of all the DetachedInstance exceptions... That's why I think
I'm doing something [deeply] wrong ( I think I have a core "concept
error")
I have designed my application in the following way:
The idea is to have my mapped classes (classes serialized in a MySQL
table) and for each class I would have a "manager" that contains a
bunch of methods that deal with the database.
Let's say I have:
class Foo(Database.Base):
_id = Column("id", Integer, primary_key=True, key="id")
_name = Column("name", String(50))
Then I will also have a "FooManager" like this:
----------------------------------------
from myLibraries.foos import Foo
class FooManager(object):
@classmethod
def getById(cls, idParam, relationshipsToPreLoad=None):
retval = None
if relationshipsToPreLoad is None:
relationshipsToPreLoad =
DatabaseUtils.getRelationships(Foo.Foo)
session = Database.Session()
try:
if (relationshipsToPreLoad):
retval = session.query(Foo.Foo).options(*
[joinedload_all(relationshipToPreLoad)
for relationshipToPreLoad in
relationshipsToPreLoad]).get(int(idParam))
else:
retval =
session.query(Foo.Foo).get(int(idParam))
session.commit()
except exc.ResourceClosedError, err:
log.debug('::getById > Manager %s > Got exception "%s".\
Probably the request was canceled by
the user' % (cls.__name__, err))
finally:
session.close()
pass
return retval
@classmethod
def update(cls, element):
if isinstance(element, Foo.Foo):
session = Database.Session()
try:
element = session.merge(element)
session.commit()
finally:
session.close()
log.debug("::update> Updated %s with id==%s" %
(element.__class__.__name__, element.id))
return element
else:
raise TypeError("Received parameter %s of type %s when
expecting
%s" % (element, type(element), Foo.Foo.__classname__))
return None
----------------------------------------
So when, in another part of the application I want to load the element
Foo.Foo with id 6, I could just do:
myFooInstance = myLibraries.foos.FooManager.FooManager.getById(6)
print myFooInstance.id
print myFooInstance.name
Or, if I create a new Foo instance, I can do:
newFoo = Foo.Foo()
newFoo.name = "fooname"
myLibraries.foos.FooManager.FooManager.update(newFoo)
and the "newFoo" will be added to the database.
The "Database" module is just a few lines long and contains the session maker:
---- Database.py -----
DSN = "mysql://mysqluser:*******@localhost/ev?charset=utf8"
engine = create_engine(DSN)
Session = scoped_session(sessionmaker(bind=engine))
Base = declarative_base()
-----------------------------------------------
The whole idea is separating the access to the database from the
classes itself (the manager's idea is something like "go to the
database, do your thing, give me a regular python class", but it's not
working fine. I've been able to find some workarounds but it's still
not working fine. Do you think this "Class/ ClassManager" type of
implementation a good idea? As you can see, in every method of the
manager I create a new instance of the session, do whatever and close
it. Is that the way the session is supposed to be used? Should the
session be global? (I can create an instance in Database.py)
I saw this:
http://groups.google.com/group/sqlalchemy/browse_thread/thread/f53f98cbbaee7b2b/14c3d8c7229ba0bc?lnk=gst&q=DetachedInstanceError#14c3d8c7229ba0bc
which seems to detail a similar problem. I tried both methods detailed
there, but I'm still getting detached instances errors. And it seems
to me that it recommends not to use detached instances (which, as far
as I understand, is what I'm trying to do here)
Any hint will be deeply appreciated. Thank you everyone!
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