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New SQLAlchemy session created when referenced in views package
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Danny Im  
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 More options Sep 5 2012, 4:44 pm
From: Danny Im <ior...@gmail.com>
Date: Wed, 5 Sep 2012 13:44:47 -0700 (PDT)
Local: Wed, Sep 5 2012 4:44 pm
Subject: New SQLAlchemy session created when referenced in views package
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Hi,

I have an issue where if I import a sqlalchemy session and try issuing
queries with it I'll get this error:

UnboundExecutionError: Could not locate a bind configured on mapper
Mapper|MyModel|models, SQL expression or this Session

This only happens from within a package within the application, as I've put
all of my views related files into a python package in order to better
organize my views.

I've tested this with view functions that reside on the 'top level' app
directory (e.g. app/views.py) and any view functions within that file don't
raise the above exception.

Here are some code bits that I think are relevant:

In /app/models.py:
DBSession =
scoped_session(sessionmaker(extension=ZopeTransactionExtension(),
autoflush=False))
def initialize_sql(engine):
    DBSession.configure(bind=engine)
    Base.metadata.bind = engine
    Base.metadata.create_all(engine)

In /app/views.py:
from models import DBSession, MyModel
session = DBSession()
session.query(MyModel)

In /app/view_dir/stuff.py:
import os
import sys
sys.path.append(os.path.abspath(os.path.join(os.path.dirname(__file__),
'..', '..', 'app')))
from models import DBSession, MyModel
session = DBSession()
session.query(MyModel)

I've searched for what might be causing the above error, and many results
suggest that the session was not bound to a database.  I tested this by
issuing print statements of the DBSession, and found that the DBSession
object is indeed different when it is imported to /app/view_dir/stuff.py,
and the engine bound to it is not set.

I've been able to reproduce this behavior by creating a new pyramid project
using the sqlalchemy scaffold, creating a package to store view files, and
creating view functions that issue queries via a sqlalchemy session object
imported from the models.py file.

Any ideas on why this is happening?  

Thanks!


 
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Jonathan Vanasco  
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 More options Sep 5 2012, 8:50 pm
From: Jonathan Vanasco <jonat...@findmeon.com>
Date: Wed, 5 Sep 2012 17:50:21 -0700 (PDT)
Local: Wed, Sep 5 2012 8:50 pm
Subject: Re: New SQLAlchemy session created when referenced in views package
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scoped_session returns a session, so to me i see:

/app/models.py:
   creates DBSession instance via scoped_session
   initialize_sql initializes it

/app/views / etc :
   imports the DBSession from /app/models, then calls it?  i think that's
where an issue may be.

have you looked into the SqlAHelper package
? https://github.com/Pylons/SQLAHelper

it's in a freeze mode, and abandoned, but it still works.


 
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Michael Merickel  
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 More options Sep 5 2012, 8:53 pm
From: Michael Merickel <mmeri...@gmail.com>
Date: Wed, 5 Sep 2012 19:52:31 -0500
Local: Wed, Sep 5 2012 8:52 pm
Subject: Re: New SQLAlchemy session created when referenced in views package
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It appears from your example code as if you're using the session from
module-scope. This is bad practice because you can't ensure that your
initialize_sql() function has executed before that code is run (when
the module is imported).


 
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