[Zope-dev] Re: SQLAlchemy integration experiment

[Laurence Rowe]

Martin Aspeli wrote:
> Martijn Faassen wrote:
>> Martin Aspeli wrote:
>>> Brian Sutherland wrote:
>> [snip]
>>>> For some reason this raises a warning bell in my head. I keep on
>>>> thinking: this is zope, the session is a classic case for a utility, we
>>>> should be getting it in views by an interface.
>>> FWIW, I had the same though.
>>>
>>> I think there's a trade-off here: we can use patterns that SQLAlchemy 
>>> and Pylons and others use directly (use a "global" that isn't 
>>> actually global) or we can use patterns that are ubiquitous in Zope 
>>> (look up utilities by interface).
>>>
>>> To my mind, the latter is better because it makes us internally 
>>> consistent, and because it promotes one of the formalisms that IMHO 
>>> makes Zope 3 easier to work with.
>>
>> The former integrates smoothly with SQLAlchemy.
> 
> Sure, that's why I said "trade-off". :)
> 
>> It also is closer to the SQLAlchemy documentation.
> 
> This is a good point. But is it "the same" or "almost the same"? If it's 
> the latter, the explicitly different is better than faux identity.
> 
>> It's also quite likely that someone writing a larger application that 
>> does use the interface lookup pattern will get bored and write 
>> something like:
>>
>> def Session():
>>     return component.getUtility(ISession)
> 
> Mmmm, I'm not so sure, but maybe.
> 
>> The Zope component architecture is not about seeing explicit calls 
>> into it everywhere. That's not the point of it. The point of it is 
>> about making applications more flexible by allowing people to plug in 
>> components. My approach allows you to do that.
> 
> Sure, but I also think that the CA has given us a few very basic, very 
> flexible idioms (adapters, utilities, subscribers) that permeate any and 
> all zope 3 applications. Internal consistency is a very good thing in a 
> framework.
> 
> A not-quite-global Session variable is another pattern to achieve what 
> we do elsewhere with global unnamed utilities when we use them as 
> effective singletons. Having two patterns to do the same thing is not good.
> 
> Put the question another way - a new user may ask, "why don't I have a 
> global thing with a capital first letter like Session to look up other 
> singletons?".
> 
>> Anyway, the balance can come out somewhere else. People are free to 
>> write their own integration approaches, it's just that mine is 
>> actually   about trying to make exactly this pattern work in the first 
>> place. Then when I succeed people want it changed. :) Anyway, no 
>> surprise: I knew that some want other patterns, and I'll be curious to 
>> see the other approaches as well.
> 
> I'm not saying it's wrong, and I do think "natural" SQLAlchemy is a 
> strong selling point, since I'd wager there are more SQLAlchemy users 
> than Zope 3 users out there. I just think we need to be careful about 
> what patterns we promote and how it fits with the rest of our "story".

I don't think scoped sessions really map well to a utility:

    session = getUtility(ISession)()

is just too ugly. Better to stick with Session() or ISession(context)

Laurence