Re: [SQLObject] Upgrading Existing Instances to New Derived Classes

[Asterio G. <ni...@ya...>]

...
> 
> What puzzles me about the scenario that you've set
> up is that any code
> that references MyExistingBaseClass will have to
> change to reference
> MyNewDerived class, which seems painful.
> 

Let me explain better:

- The idea is to be able of upgrading ONLY some
instances
- Existing external application using (a) tables keep
working (adding 'child_name' column in (a) when
necessary )
- From new python applications you can use additional
properties/codes to preforms avanced features using
(b) classes, keeping separately data stored in (a) and
(b) tables
- You can manage as many (b) different classes as you
need derived from a single (a). A Simply ALTER TABLE
to add new attributes in (a) doesn't work
- External application can insert new (a) instances in
DB, so I need to check and fire up a function to
detect and upgrade this (a) records to appropriate (b)
classes

Here is an example

class basicWorkUnit (InheritableSQLObject):	# This is
(a) class
  """Compatible with existing application"""
	....

class WorkUnit ( basicWorkUnit ):  		# This is a (b)
class
  """Additional attribute and functions  ..."""
	....

class Meeting ( basicWorkUnit ):  		# This is other
(b) class
  """Additional attribute and functions  ..."""
	....


> My solution is this:
> 
> 1) Keep track of what schema version is in use
> (somewhere in the database)
> 2) Change the existing class to match the new
> definition
> 3) when the program fires up, make sure that the
> database schema is in
> sync with the code
> 4) Write an upgrade script that performs appropriate
> alter tables and
> selects if the database schema is an older version
> than the code
> 

I agree with you upgrading whole DB schema, but
SQLObject can do (4) step directly from my class
definition, I prefer keep away errors when maintain
two 'sources' and write a check code similar to:

for intance in basicWorkUnit.select( some_filters  AND
 child_name <> 'WorkUnit'):
	upgradeInstance(instance,WorkUnit)

for intance in basicWorkUnit.select(
some_other_filters  AND  child_name <> 'Meeting'):
	upgradeInstance(instance,Meeting)


Regards,
--
Asterio


	
	
		
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