Comments on: A few rules I try to follow with TurboGears

By: matt

matt — Mon, 17 Mar 2008 18:32:15 +0000

Christopher Arndt: Most of the time, I don’t care about coercing into a list in the controller before the template because like you point out, it’s the same cost if I do it in the controller vs the template.

However, in templates where I iterate through the results twice, it does make sense to coerce the selectResults into a list. Otherwise, I’ll run the exact same query twice:


In [24]: User.select(User.q.user_name.startswith('Z'))

Out[24]: 

In [25]: list(_24)
 1/Select  :  SELECT tg_user.id, tg_user.modifieddate, tg_user.createddate, tg_user.supervisor_id, tg_user.first_name, tg_user.last_name, tg_user.v2organization_id, tg_user.participationstatus_id, tg_user.user_name, tg_user.email_address, tg_user.password, tg_user.mobilenumber, tg_user.homenumber FROM tg_user WHERE ((tg_user.user_name) LIKE ('Z%'))
 1/QueryR  :  SELECT tg_user.id, tg_user.modifieddate, tg_user.createddate, tg_user.supervisor_id, tg_user.first_name, tg_user.last_name, tg_user.v2organization_id, tg_user.participationstatus_id, tg_user.user_name, tg_user.email_address, tg_user.password, tg_user.mobilenumber, tg_user.homenumber FROM tg_user WHERE ((tg_user.user_name) LIKE ('Z%'))

Out[25]: []

In [26]: list(_24)
 1/Select  :  SELECT tg_user.id, tg_user.modifieddate, tg_user.createddate, tg_user.supervisor_id, tg_user.first_name, tg_user.last_name, tg_user.v2organization_id, tg_user.participationstatus_id, tg_user.user_name, tg_user.email_address, tg_user.password, tg_user.mobilenumber, tg_user.homenumber FROM tg_user WHERE ((tg_user.user_name) LIKE ('Z%'))
 1/QueryR  :  SELECT tg_user.id, tg_user.modifieddate, tg_user.createddate, tg_user.supervisor_id, tg_user.first_name, tg_user.last_name, tg_user.v2organization_id, tg_user.participationstatus_id, tg_user.user_name, tg_user.email_address, tg_user.password, tg_user.mobilenumber, tg_user.homenumber FROM tg_user WHERE ((tg_user.user_name) LIKE ('Z%'))

Out[26]: []

However, what I really care about is making sure that I don’t follow any multiplejoins or related joins once I’m already in the template. So instead of doing something like this in my template:


    for employee in employees:
        for task in employee.tasks: # tasks are a related join, and this does a query every time.

I’ll build a dictionary in my controller that maps employees to tasks:


    for employee in employees;
        for task in employee_tasks[employee]

The rule of thumb about making sure everything is already out of the database before I get to the template is just a simple way of avoiding the previous two scenarios.

By: Christopher Arndt

Christopher Arndt — Mon, 17 Mar 2008 18:18:41 +0000

Your first rule seems very tough to meet. Wouldn’t that mean that you need to flatten any generator for select results to a list and also do the same for any attribute capsuling a 1:n or n:m-relation?

I don’t see the advantage of this. If you want to see the queries executed by your page controller, turn on SQLObject/SQLAlchemy debugging. Anyway, there’s no real difference between controller and template processing in this respect. They both happen in your controller method, only the template rendering is hidden in the “expose” decorator.

By: matt

matt — Mon, 17 Mar 2008 15:52:12 +0000

Chris, thanks for the comments! If I understand you right, instead of


    redbirds = classmethod(partial(by_color, color='red'))
    bluebirds = classmethod(partial(by_color, color='blue'))

I could do:


    by_color = classmethod(by_color)
    redbirds = partial(by_color, color='red')
    bluebirds = partial(by_color, color='blue')

I haven’t tried it out, but I like that I don’t need to keep using classmethod over and over again in your approach.

By: Chris

Chris — Mon, 17 Mar 2008 12:44:27 +0000

Concerning the sidenote, this is because an unbound classmethod is indeed not callable (it only wraps a callable that you need to __get__ first). A workaround would be adding by_color=classmethod(by_color) at the end of the class declaration.