Re: Rename Table limiting the down of the system

On 15/03/2017 7:12 PM, Stefano Tranquillini wrote:
> Hi people.
>
> I've to do a migration for the database, specifically I've to rename a
> table.
>
> To do so I've a RunPython that runs this sql statemnet
> `ALTER TABLE "old_table" RENAME TO "new_table"`
> First question: should I use
> https://docs.djangoproject.com/en/1.8/ref/migration-operations/#renamemodel
> , does it do the same?

I believe it does the same ... certainly worked for me with Postgres.
Make an empty migration.

operations = [

migrations.RenameModel('Oldname', 'Newname'),

migrations.AlterModelTable('Newname', 'appname_newname'),

]


> Will these operation update all the refrences in the code, such that
> django will keep working without problems?

Definitely not.

The process I used was ...

1. Change the model name in your code

2. Run the dev server and fix everything which barfs. I think unit
testing creates its test database from your models rather than by
copying the database so unit tests should be fixable with
search/replace. However, any fixtures involved will undoubtedly spoil
things. ymmv.

3. Migrate and resolve any remaining issues.

Rollout to the two servers sounds like a transation nightmare to me. I
think I'd say some things are possible but expensive because you have to
set up duplicated systems for practicing such rollouts or you down the
system for a short time to make sure the data is protected.

ALSO

I have had migration squashing issues (circular references) since then.
It may or may not be the reason but I think in hindsight I would squash
all migrations prior to renaming the table.

Good luck

Mike

>
> Now, I'm using postgres and I've two servers using the same DB, let's
> call them A and B. when I'll rename the table the code will break, so
> the procedure will be:
>
> - stop A
> - update A
> - migrate
> - stop B
> - start A
>
> The problem is, that I've to be sure that migrations are atomic and
> have a transaction that locks the DB (or that table). Reading the
> documentation I found:
> /On databases that do support DDL transactions (SQLite and
> PostgreSQL), |RunPython| operations do not have any transactions
> automatically added besides the transactions created for each
> migration./ -
> https://docs.djangoproject.com/en/1.10/ref/migration-operations/ (I'm
> using 1.8)
> If i go with renameModel, will it be in a single transaction?
>
> What I want to avoid is to stop both A and B to perform migrations.
> It would be better to do the migration and then stop B, this should
> have a down that is smaller compared to stopping all services and the
> run them again.
>
> any suggestion?
>
> Ciao
>
>
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