On Mar 9, 2011, at 4:57 AM, Yang Zhang wrote:
> I have a model with a field:
>
> class Obj(Base):
> ...
> meta = Column(PickleType)
>
> For some reason, when I do
>
> obj = session.query(Obj).get(id)
> obj2 = session.query(Obj2)....
>
> I get a dirty UPDATE being attempted on the first object (noticed this
> due to read-only access to a Postgresql DB):
>
> InternalError: (InternalError) transaction is read-only
> 'UPDATE intel.opportunity_runs SET meta=%(meta)s WHERE
> intel.opportunity_runs.id = %(intel_opportunity_runs_id)s' {'meta':
> <psycopg2._psycopg.Binary object at 0x2f8fe18>,
> 'intel_opportunity_runs_id': '21da68c8-499d-11e0-a5c9-12313f00dc12'}
>
> I'm using 0.6.4. I haven't been able to repro this issue outside the
> app. Just wondering if there's any high-level tips on how to best go
> about debugging this. Thanks in advance.
It would appear that the comparison of the pickled data is coming up as
"modified", which is a common occurrence. To debug this, you'd do a pdb or
print statement after "obj" is loaded, assuming that's the one being marked
dirty, then do an "obj in session.dirty" which would probably return True, then
if still unsure that it's "meta" do an "attributes.get_history(obj, "meta")"
and inspect the history object. Within get_history() the comparison is
actually performed. By default the "==" operator is used, as defined by the
"compare_values()" method of the column's type object. The History object
returns a tuple with ([new value], (), [previous value]), the "previous value"
being a copy of the original. If "new_value != previous_value", that is the
issue.
Going forward, just generally, if you see UPDATEs coming out for no reason and
you see a PickleType, its already obvious that this is the issue. "mutable"
types are headed towards deprecation in favor of the "mutable scalars"
extension in 0.7. The strategies, in order of preference are:
1. set mutable=False on the PickleType. Your CPU will thank you as
"mutable=True" is extremely inefficient.
2. If "in place mutation detection" is definitely needed, if possible (I know,
its probably not, just go to #3), upgrade to 0.7 and use the new mutable
scalars extension to detect in-place change events.
3. add a "comparator" function to your PickleType, or implement an __eq__()
method on the object that you're pickling.
>
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