[storm] Update or add pattern with Storm

[Jamu Kakar]

Hi,

Gerdus van Zyl wrote:
> I currently do this:
> store = getStore()
> rcount = store.find(customer,customer.lastname==u"Sparrow").count()
> exists = (rcount > 0)
> print exists
> 
> Which is simple as dirt, what code do you use? And why do you say it's 
> not efficient?

According to somewhat dated information[1] COUNT can be slow on
PostgreSQL because it does table scans.  It hints at improvements in
8.2 that might make the issues explained go away.  Nonetheless, I
have heard of people having performance problems with COUNT in the
recent past, so I avoid it unless I actually need the row count.  I
primarily use PostgreSQL so I don't know anything about COUNT with
other database backends.

I use ResultSet.any to produce the same result as your code snippet
above, to avoid using COUNT:

exists = bool(store.find(Customer, Customer.last_name == u"Sparrow").any())

To solve the original poster's issue, where one needs to determine
if an object exists and then create or update it, I'd use something
like:

customer = store.find(Customer, Customer.last_name == u"Sparrow").one()
if not customer:
     customer = store.add(Customer(u"Jack", u"Sparrow"))
else:
     customer.first_name = u"Jack"
     customer.last_name = u"Sparrow"

You need to use a query that will only ever return 0 or 1 row when
you use ResultSet.one.  If more than 1 row is returned a NotOneError
will be raised.

Thanks,
J.

[1] 
http://wiki.postgresql.org/wiki/Introduction_to_VACUUM,_ANALYZE,_EXPLAIN,_and_COUNT