On Dec 15, 2011, at 2:13 PM, Kent wrote: > On 12/15/2011 1:31 PM, Michael Bayer wrote: >> Are there reasons one need to avoid referencing unloaded relationships from >> within before_upate()? (I can't recall the exact problem I've had in the >> past with that at the moment.) > > Thanks for all the input. Regarding the issue of referencing a relationship > from within before_update()/before_insert(), etc. I looked up my previous > problem and it is related to this code from strategies.py: > > class LazyLoader() > ... > def _load_for_state(self, state, passive): > ... > if self.use_get: > if session._flushing: > get_attr = instance_mapper._get_committed_state_attr_by_column > else: > get_attr = instance_mapper._get_state_attr_by_column > > If we are in the middle of a flush, we don't attempt a query but rather use > the committed state. This caused me problems when the committed state of the > relationship was "None" (for example, not loaded yet?) > > For my own understanding, what implications does this have? It kind of > freaked me out because I interpreted that as meaning "I'm not sure if I can > rely upon unloaded relationships at all from within these mid-flush event > methods..." because I certainly don't want to be using stale, old data. Is > there a threat of this, or only of it sometimes being "None"?
so this is we're inside of a flush, and suppose we need to get at someobject.some_related specifically to target what's present in the database - if you've changed the value of someobject.some_related_id, that isn't flushed yet - we need some_related relative to the some_related_id that was loaded from the database. There's still potentially a query run to get at some_related, just the foreign key value we use from the parent is different - it's looking at the persistent value. To get more specific, comment out the conditional and just make it do the non-committed, then run the orm unit tests. The tests that fail will be the ones that illustrate the use case. -- You received this message because you are subscribed to the Google Groups "sqlalchemy" group. To post to this group, send email to sqlalchemy@googlegroups.com. To unsubscribe from this group, send email to sqlalchemy+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/sqlalchemy?hl=en.
