Hi Mike
I would like a little insight into the session object, and the declarative_base
class.
I have a process running many threads, where each thread may be connected to
potentially a different engine/database. If the database connection between 2
or more threads is the same, then they will share the same engine. However,
they each have their own MetaData objects.
There is a global sessionmaker() that has no binding at that time. When each
thread creates its OWN session, then it processes mySession =
Session(bind=myThreadsEngine).
The Engines and MetaData part has worked perfectly for years, using basic
queries like Table(’some_table', threadMetaData,
autoload=True).select().execute().fetchall(). etc.
I’ve started to use the ORM more now, and am using the relationships between
the objects. However, I’m hitting and issue that appears to centre around some
shared registry or class variables or something that is causing a conflict.
I’ve made it so each THREAD has is own Base =
declarative_base(metadata=theSessionsMetaData)
Then, classes are mapped dynamically based on this new Base, and the columns
are autoload’ed. Again, this is working - sometimes. There’s some
random-like problem that mostly means it does not work when I do a
mySession.query(myMappedClassWithRelationships) and I get the following
exception being raised;
File "C:\Python27\lib\site-packages\dap-2.1.2-py2.7.egg\dap\db\dbutils.py",
line 323, in DAPDB_SetColumns
query = session.query(mappedClass).filter_by(**whereCriteria)
File "build\bdist.win32\egg\sqlalchemy\orm\session.py", line 1260, in query
return self._query_cls(entities, self, **kwargs)
File "build\bdist.win32\egg\sqlalchemy\orm\query.py", line 110, in __init__
self._set_entities(entities)
File "build\bdist.win32\egg\sqlalchemy\orm\query.py", line 120, in
_set_entities
self._set_entity_selectables(self._entities)
File "build\bdist.win32\egg\sqlalchemy\orm\query.py", line 150, in
_set_entity_selectables
ent.setup_entity(*d[entity])
File "build\bdist.win32\egg\sqlalchemy\orm\query.py", line 3446, in
setup_entity
self._with_polymorphic = ext_info.with_polymorphic_mappers
File "build\bdist.win32\egg\sqlalchemy\util\langhelpers.py", line 754, in
__get__
obj.__dict__[self.__name__] = result = self.fget(obj)
File "build\bdist.win32\egg\sqlalchemy\orm\mapper.py", line 1891, in
_with_polymorphic_mappers
configure_mappers()
File "build\bdist.win32\egg\sqlalchemy\orm\mapper.py", line 2768, in
configure_mappers
mapper._post_configure_properties()
File "build\bdist.win32\egg\sqlalchemy\orm\mapper.py", line 1708, in
_post_configure_properties
prop.init()
File "build\bdist.win32\egg\sqlalchemy\orm\interfaces.py", line 183, in init
self.do_init()
File "build\bdist.win32\egg\sqlalchemy\orm\relationships.py", line 1628, in
do_init
self._process_dependent_arguments()
File "build\bdist.win32\egg\sqlalchemy\orm\relationships.py", line 1653, in
_process_dependent_arguments
setattr(self, attr, attr_value())
File "build\bdist.win32\egg\sqlalchemy\ext\declarative\clsregistry.py", line
293, in __call__
(self.prop.parent, self.arg, n.args[0], self.cls)
InvalidRequestError: When initializing mapper
Mapper|SalesDocument|rm_dt_documents, expression
'SalesDocumentLine.parentID==SalesDocument.id' failed to locate a name ("name
'SalesDocumentLine' is not defined"). If this is a class name, consider adding
this relationship() to the <class 'dap.db.dbmanager.SalesDocument'> class after
both dependent classes have been defined.
I understand what this is trying to tell me, however, the classes ARE defined.
Sometimes the code works perfectly, but mostly not. If I have ONE Thread
working and then start up another using exactly the same code, then it will
probably NOT work but more importantly, the one that WAS working then dies with
the same error. Clearly something somewhere is shared - I just can’t find out
what it is, or how I can separate the code further.
In summary;
one global sessionmaker()
global Session=sessionmaker()
each thread (for the example here) shares an Engine
each thread has it’s OWN session from mySession = Session(bind=e)
each thread has it’s own Base created from
declarative_base(metadata=threadsMetaData)
I’m declaring two classes in this example. SalesDocument and SalesDocumentLine.
The relationships are set up and ‘can’ work on occasion.
In that error, where exactly are they not ‘defined’. I’ve looked in
Base.decl_class_registry and both those names are there! Where else do they
need to be to be considered ‘declared'?
Any pointers as to the error of my ways would be most appreciated. :-)
Cheers
Warwick
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