[sqlalchemy] Re: Use of new_instance() in the user-defined-state branch? - Phillip J. Eby - com.googlegroups.sqlalchemy

17 messages in com.googlegroups.sqlalchemy[sqlalchemy] Re: Use of new_instance(...

From	Sent On	Attachments
Phillip J. Eby	27 Mar 2008 11:51
jason kirtland	27 Mar 2008 12:01
Michael Bayer	27 Mar 2008 12:10
Michael Bayer	27 Mar 2008 12:42
Phillip J. Eby	27 Mar 2008 14:06
jason kirtland	27 Mar 2008 14:25
Phillip J. Eby	27 Mar 2008 17:38
jason kirtland	27 Mar 2008 18:08
Phillip J. Eby	27 Mar 2008 21:54
Michael Bayer	28 Mar 2008 06:44
Phillip J. Eby	28 Mar 2008 07:57
Michael Bayer	28 Mar 2008 08:37
jason kirtland	28 Mar 2008 08:55
Phillip J. Eby	28 Mar 2008 09:13
jason kirtland	28 Mar 2008 09:26
Michael Bayer	28 Mar 2008 09:28
Michael Bayer	28 Mar 2008 09:53

Subject:	[sqlalchemy] Re: Use of new_instance() in the user-defined-state branch?
From:	Phillip J. Eby (pj...@telecommunity.com)
Date:	03/28/2008 07:57:37 AM
List:	com.googlegroups.sqlalchemy

At 09:45 AM 3/28/2008 -0400, Michael Bayer wrote:

On Mar 28, 2008, at 12:55 AM, Phillip J. Eby wrote:

Sadly, about the only way for me to implement that without code duplication will be to temporarily change the item's __class__ to a subclass with an empty __init__ method. Unless there's a way to change the generated __init__ method to take an extra flag or check a per-thread variable to skip the bits you don't want? What are the bits you don't want run, anyway? That is, what specifically mustn't happen?

an end-user's __init__ method is not run when the object is loaded from the DB since the ORM is going to populate its state explicitly. its for similar reasons that pickle.loads() doesn't call __init__. Its a common use case that someone's class needs to be constructed differently when it is newly created vs. when it is re-populated from the DB.

The usual method we have of modifying this behavior is to use a MapperExtension where you implement create_instance(). Then you can build the object any way you want, using __init__, whatever. Why is that not an option here ? ( or has it just not been mentioned ?)

Because Jason said this:

At 06:08 PM 3/27/2008 -0700, jason kirtland wrote:

Phillip J. Eby wrote:

At 02:26 PM 3/27/2008 -0700, jason kirtland wrote:

new_instance creates an instance without invoking __init__. The ORM uses it to recreate instances when loading from the database. new_instance can be added to InstrumentationManager as an extension method... The ORM doesn't care how empty instances are manufactured so long as they can be created without initialization arguments, e.g. a no-arg constructor.

Does that mean that no attributes must be set from

new_instance(), either?

You should be able to set whatever you like there.

So... new_instance() could literally just be a call to 'self.class_()', with no other behavior, as long as the constructor requires no arguments? The modified __init__ that SA inserts won't be a problem there?

Sorry, I should have included more detail. You don't want to trigger the logic in SA's __init__ decorator or call the user's __init__. The ORM policy is not to __init__ on load, and will soon support a symmetric "__on_load__" type of hook that the ORM can call post-__new__ when reconstituting instances.

So, which one of you is right? :)