On Jun 17, 2011, at 8:41 AM, Moch Ramis wrote:
> 2011/6/15 Michael Bayer <mike...@zzzcomputing.com>
> Assuming you're talking about the ORM. You can of course emit the exact SQL
> you refer to any time using execute() from an engine, connection, or session.
> The ORM works within a transaction at all times. so an expression like:
>
> myobject = MyObject(id=5)
> myobject = Session.merge(myobject)
> Session.commit()
>
> will perform the equivalent steps, subject to the transaction isolation in
> effect.
>
> I'm not sure this will do the trick as i don't know if the object i'm looking
> for already exist or not. thus, i don't have it's id ( i don't even know if a
> range of id is free or not).
OK then similar,
myobject = s.query(MyClass).filter(...).first()
if myobject is None:
# ... etc
>
>
> To lock the row during the SELECT, you can use
> query.with_lockmode(True).filter(criterion) to SELECT the row with FOR
> UPDATE, will return None if the row does not exist.
>
> I'm not sure a lockMode is necessary.
right actually this doesn't apply here since we are talking about an INSERT not
an UPDATE, sorry.
> However, I'm far from being an expert or to clearly know the internal
> mechanisms of DBMS, but reading the Deadlock example in the part 12.3.3 of
> this page of the postgresql documentation, I thought, that sending a bunch of
> requests in one transaction would allow to do it as if it was all at once,
> avoiding any conflicts that could occurs if two almost identical requests
> were overlapping (like one executing its first statement, the other the
> second ect..).
yah Session is in a transaction which completes when you call rollback() or
commit(). I don't think there's any additional atomicity to an "IF NOT
EXISTS...SELECT" type of phrase versus a SELECT then an INSERT (if there is
that would be surprising).
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