David Cramer's Blog - Latest Comments in An Identity Mapper in Django (formerly Django Singletons)

Re: An Identity Mapper in Django (formerly Django Singletons)

Tom. — Tue, 02 Dec 2008 18:47:29 -0000

This sounds wicked! Thanks.

Re: An Identity Mapper in Django (formerly Django Singletons)

dailycad — Tue, 02 Dec 2008 16:45:07 -0000

Very useful info.. Thanks..

Re: An Identity Mapper in Django (formerly Django Singletons)

David Cramer — Tue, 02 Dec 2008 15:21:50 -0000

Ah if you are correct then I will be renaming the project. I haven't done much research (in terminology), but had assumed it was correct via the ticket on Django.

Re: An Identity Mapper in Django (formerly Django Singletons)

Steingrim — Tue, 02 Dec 2008 15:10:37 -0000

I think calling this Singletons is quite misleading. The Singleton Pattern is used to enforce a single instance of a class; not a single instance of a business entity. Identity Map is the correct naming but note Fowler's emphasis on using one Identity Map per transaction.

Re: An Identity Mapper in Django (formerly Django Singletons)

David Cramer — Tue, 02 Dec 2008 14:34:51 -0000

Personally, I plan to use it throughout the codebase to prevent some data issues. The one area this doesn't affect right now (due to complications we had with it at Curse) is serialization. It will ignore the instance caching when you unserialize objects (such as from the cache).

Re: An Identity Mapper in Django (formerly Django Singletons)

David Cramer — Tue, 02 Dec 2008 14:31:28 -0000

I think per query is a bit out of scope for what I want to achieve. Doing this would require the queryset/manager to be aware (to some extent) of the singleton model. This wouldn't quite achieve the same performance benefit.

The biggest thing about this, while it IS an obvious performance boost, its not the sole intention of the project. Having unique instances of an object in memory can solve a lot of other issues as well.

Re: An Identity Mapper in Django (formerly Django Singletons)

Sam Toriel — Tue, 02 Dec 2008 11:45:27 -0000

^^ My questions exactly

Re: An Identity Mapper in Django (formerly Django Singletons)

Rob Hudson — Tue, 02 Dec 2008 11:42:49 -0000

Could there be a way to enable this per query? In your example, it's obvious that creating only one instance per category is optimal, but are there cases where it may not be what you want?

Re: An Identity Mapper in Django (formerly Django Singletons)

Tom Davis — Tue, 02 Dec 2008 09:21:24 -0000

So, two questions.

1. Per your explanation, it wasn't strictly necessary to define Article as a SingleModel, correct?

2. Given the obvious benefits, are there any reasons not to use this class for all models? Am I missing something crucial here?