Performance improvements for the Site Configurations when completion is enabled

[OmarIthawi]

This pull request does two changes:

• Improve the performance of retrieve_last_sitewide_block. By making the SELECT * FROM site_configuration_siteconfiguration query only when needed.
• Make the URL returned from retrieve_last_sitewide_block site aware. By removing the domain part and using relative URLs instead i.e. starting with /.

[thraxil]

Are we positive that retrieve_last_sitewide_block only gets called in a web context, where we know a relative URL will work? Ie, if the URL it generates ends up in an email, a relative one without the host/site wouldn't work.

[melvinsoft]

@OmarIthawi I'm a bit worried about how this approach affects enterprise or normal edX installations (if we want to push upstream) because is gonna pull SiteConfiguration.objects.all() on every course the user is enrolled in, every-time a user reach the dashboard.

[johnbaldwin]

Good point, @melvinsoft . A couple of questions for @OmarIthawi and/or @melvinsoft

A. Does it make sense to add a tahoe/multisite env flag for this?
B. Where is the source of 'course_org_filter'? I've looked over the SiteConfiguration class and have trouble finding the source of truth for this field

[OmarIthawi]

Great point @melvinsoft, I've missed that. I'll improve the task to be at least equal to the original behaviour, not worse.

[OmarIthawi]

Thanks @johnbaldwin, I'll reply inline:

A. Does it make sense to add a tahoe/multisite env flag for this?

Maybe but I'm trying to avoid having such configuration to avoid having too much conflicts when we merge with Open edX.

B. Where is the source of 'course_org_filter'? I've looked over the SiteConfiguration class and have trouble finding the source of truth for this field

That would be found in the SiteConfiguration.values dictionary (example):

[melvinsoft]

@OmarIthawi Thanks for putting this together, I just posted a question for you.

[OmarIthawi]

@melvinsoft I think I've fixed the issue, please take a look.

[OmarIthawi]

@thraxil

Are we positive that retrieve_last_sitewide_block only gets called in a web context, where we know a relative URL will work? Ie, if the URL it generates ends up in an email, a relative one without the host/site wouldn't work.

I have checked it and the function doesn't seem to be used anywhere other than web requests.

@melvinsoft

I'm a bit worried about how this approach affects enterprise or normal edX installations (if we want to push upstream) because is gonna pull SiteConfiguration.objects.all() on every course the user is enrolled in, every-time a user reach the dashboard.

I've fixed this and the query now runs one time at max, and only when needed.

[melvinsoft]

@OmarIthawi Thanks for the fix!