Hey Mike,
I've found and fixed (with help) my problem, but I thought I'd describe it 
here to support anyone else who hits this...
Was not a threading issue......it was a combination of:

   - data-edge case
   - bug in our code
   - bug in how SA was reporting a failure...
   
I was adding a model object to the session as follows:

rec = get_or_create(sess, CtrAtt, ctrAttSearchVals, ctrAttNewVals ) 

sess.add(rec)


next I was looking up a record that was needed as an FKEY to the CtrAtt 
record I just put in the session  (oops.....our bug)

cot = sess.query(ContribTitle).get(int(cot_gem_id))


That query was of course flushing the session...but the record sought via 
"ContribTitle" was not found  (the data-edge case)

When SA saw the FKEY integrity error, it was attempting to roll-back the 
transaction.....and it was not showing the DB Integrity error....

Somewhere in that process, SA attempted to access a null "impl" object and 
confusingly throwing the:

'NoneType' object has no attribute 'accepts_scalar_loader'

Regards,
Dewey





On Wednesday, September 16, 2015 at 6:24:47 PM UTC-5, Michael Bayer wrote:
>
> check out this case, this is one way to reproduce that:
>
>
> https://bitbucket.org/zzzeek/sqlalchemy/issues/3532/detect-property-being-assigned-to-more
>
>
>
> On 9/15/15 3:56 PM, dewey wrote:
>
> I'll see if I can reproduce this in a simple example.....it's currently in 
> a job being run every night from a Celery-Beat scheduler...... 
>
> Interestingly, just changing the data in the DB made this problem go away 
> temporarily.......we were seeing this error thrown every night on staging 
> for 5 days straight,
> But after we copied the Prod data into staging, it went away for about a 
> week.....
>
> This morning, it threw on both staging and prod......first time we've seen 
> it in Prod....
>
> Thanks for your thoughts...
> Dewey
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