Hi Jonathan,
As this raises a valid concern, it can't be "too critical" ;-) BTW,
this has been discussed multiple times in the last few months. So....
1. You are right - current Cayenne schema generation is all or nothing.
You can drop and recreate your tables but there is no "merge".
2. In the future we will definitely improve that. This is in the plans.
However there is a huge BUT, which lead us to (3)...
3. In a general case "merging" DB schema with the evolved model is not
just add/drop column. Most real life migrations between schema versions
are incomparably more complex than that. They involve moving data
within and between tables, modifying this data in the process,
intermingling DDL and DML statements, and is generally impossible to
automate.
In my work as a consultant, I've spent a fair amount of time on various
projects trying to automate or at least improve tedious database schema
upgrade process between the code versions. My conclusion is that full
generic (and most importantly - working) automation is impossible. If
you have a production DB with lots of data, expect doing manual SQL
coding. Somebody please prove me wrong on that!
On a more optimistic note, we can still *greatly* improve this
experience by providing a dumb generated "SQL patch" that compares
current DB state and current Cayenne DataMap, and generates "create
table/add column/drop column" statements without regards to the actual
data migration path... It can be saved and later manually customized
for the actual DB run. That's where we are heading.
A bit unrelated, I guess I should follow the suggestion made privately
by Michael Gentry and set up a Wiki for Cayenne, so that such
discussions could leave a trail and an actual implementation roadmap.
Thanks
Andrus
On Jul 23, 2004, at 5:20 PM, Jonathan Carlson wrote:
> I hope I didn't sound too critical in my note below. Cayenne is a
> very easy to use tool and I'm sure it will only get easier to use as
> time goes on. My thinking that Cayenne might make this really easy
> for me is a testament to how easy it is already.
>
> As I thought about it I realized that adding a column could be a
> difficult thing for Cayenne to manage if the column was mandatory
> because Cayenne does not yet allow a default value specification. I'm
> hoping that specifying a default value will be a new Cayenne feature
> soon since it seems like a useful feature and all databases (I think)
> support it at some level.
>
> Are these features "on the horizon" or "down the road further"?
> (default column specification, and smarter schema evolution)
>
> Thanks,
>
> Jonathan
>
>
>>>> "Jonathan Carlson" <Jonathan.Carlso..atun.com> 2004-07-23 1:37:07
>>>> PM >>>
> I did a quick test to see how Cayenne handled adding a new field when
> there was data already in the table. I was secretly hoping that
> Cayenne would do the right thing and generate an add column statement
> rather than a create table statement, but it didn't.
>
> To top it off, clicking on the "Generate" button resulted in a "Schema
> Generation Complete" message as if the db schema now matched what was
> in Cayenne, which it didn't because no column was added.
>
> Is this a bug or just a missing feature? What's the best way for me
> to handle this situation? Add columns manually?
>
> Thanks in advance!
>
> Jonathan
>
>
>
>
>
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