On Sep 11, 2007, at 2:25 AM, Hermann Himmelbauer wrote:
> > Hi, > I need to safely increase a numeric value via SQLAlchemy: > I have a table that has a column with numeric data. For increasing > the value, > I need to read the data, add some value and store it, so it would > look like > that: > > - select myvalue from mytable > - myvalue += 123 > - update mytable and set myvalue to 123 > > However, problems arise when two of this operations are done > simultaneously, > e.g. operation A and B: > > A - select > B - select > A - increase > B - increase > A - update > B - update > > In this case, operation A is overwritten by B. > > One viable solution would be to make the operation atomic, by e.g. > locking the > database row. Is this possible with SQLAlchemy? If yes, how? > > Or is there a better way? > we can lock rows using SELECT ... FOR UPDATE expressions. the select () construct includes a "for_update=True" flag for this, and the ORM's Query object supports a with_lockmode() method that can add FOR UPDATE semantics to the SELECT statement. the docstrings for select () and Query should explain both. --~--~---------~--~----~------------~-------~--~----~ 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 [EMAIL PROTECTED] For more options, visit this group at http://groups.google.com/group/sqlalchemy?hl=en -~----------~----~----~----~------~----~------~--~---
