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Hi all,

My two cents.

> 2) Extend alembic so that op.drop_column() does the right thing
We could, but should we?

The only reason alembic doesn't support these operations for SQLite
yet is that SQLite lacks proper support of ALTER statement. For
sqlalchemy-migrate we've been providing a work-around in the form of
recreating of a table and copying of all existing rows (which is a
hack, really).

But to be able to recreate a table, we first must have its definition.
And we've been relying on SQLAlchemy schema reflection facilities for
that. Unfortunately, this approach has a few drawbacks:

1) SQLAlchemy versions prior to 0.8.4 don't support reflection of
unique constraints, which means the recreated table won't have them;

2) special care must be taken in 'edge' cases (e.g. when you want to
drop a BOOLEAN column, you must also drop the corresponding CHECK (col
in (0, 1)) constraint manually, or SQLite will raise an error when the
table is recreated without the column being dropped)

3) special care must be taken for 'custom' type columns (it's got
better with SQLAlchemy 0.8.x, but e.g. in 0.7.x we had to override
definitions of reflected BIGINT columns manually for each
column.drop() call)

4) schema reflection can't be performed when alembic migrations are
run in 'offline' mode (without connecting to a DB)
...
(probably something else I've forgotten)

So it's totally doable, but, IMO, there is no real benefit in
supporting running of schema migrations for SQLite.

> ...attempts to drop schema generation based on models in favor of migrations

As long as we have a test that checks that the DB schema obtained by
running of migration scripts is equal to the one obtained by calling
metadata.create_all(), it's perfectly OK to use model definitions to
generate the initial DB schema for running of unit-tests as well as
for new installations of OpenStack (and this is actually faster than
running of migration scripts). ... and if we have strong objections
against doing metadata.create_all(), we can always use migration
scripts for both new installations and upgrades for all DB backends,
except SQLite.

Thanks,
Roman

On Sat, Feb 1, 2014 at 12:09 PM, Eugene Nikanorov
<enikanorov at mirantis.com> wrote:
> Boris,
>
> Sorry for the offtopic.
> Is switching to model-based schema generation is something decided? I see
> the opposite: attempts to drop schema generation based on models in favor of
> migrations.
> Can you point to some discussion threads?
>
> Thanks,
> Eugene.
>
>
>
> On Sat, Feb 1, 2014 at 2:19 AM, Boris Pavlovic <bpavlovic at mirantis.com>
> wrote:
>>
>> Jay,
>>
>> Yep we shouldn't use migrations for sqlite at all.
>>
>> The major issue that we have now is that we are not able to ensure that DB
>> schema created by migration & models are same (actually they are not same).
>>
>> So before dropping support of migrations for sqlite & switching to model
>> based created schema we should add tests that will check that model &
>> migrations are synced.
>> (we are working on this)
>>
>>
>>
>> Best regards,
>> Boris Pavlovic
>>
>>
>> On Fri, Jan 31, 2014 at 7:31 PM, Andrew Lazarev <alazarev at mirantis.com>
>> wrote:
>>>
>>> Trevor,
>>>
>>> Such check could be useful on alembic side too. Good opportunity for
>>> contribution.
>>>
>>> Andrew.
>>>
>>>
>>> On Fri, Jan 31, 2014 at 6:12 AM, Trevor McKay <tmckay at redhat.com> wrote:
>>>>
>>>> Okay,  I can accept that migrations shouldn't be supported on sqlite.
>>>>
>>>> However, if that's the case then we need to fix up savanna-db-manage so
>>>> that it checks the db connection info and throws a polite error to the
>>>> user for attempted migrations on unsupported platforms. For example:
>>>>
>>>> "Database migrations are not supported for sqlite"
>>>>
>>>> Because, as a developer, when I see a sql error trace as the result of
>>>> an operation I assume it's broken :)
>>>>
>>>> Best,
>>>>
>>>> Trevor
>>>>
>>>> On Thu, 2014-01-30 at 15:04 -0500, Jay Pipes wrote:
>>>> > On Thu, 2014-01-30 at 14:51 -0500, Trevor McKay wrote:
>>>> > > I was playing with alembic migration and discovered that
>>>> > > op.drop_column() doesn't work with sqlite.  This is because sqlite
>>>> > > doesn't support dropping a column (broken imho, but that's another
>>>> > > discussion).  Sqlite throws a syntax error.
>>>> > >
>>>> > > To make this work with sqlite, you have to copy the table to a
>>>> > > temporary
>>>> > > excluding the column(s) you don't want and delete the old one,
>>>> > > followed
>>>> > > by a rename of the new table.
>>>> > >
>>>> > > The existing 002 migration uses op.drop_column(), so I'm assuming
>>>> > > it's
>>>> > > broken, too (I need to check what the migration test is doing).  I
>>>> > > was
>>>> > > working on an 003.
>>>> > >
>>>> > > How do we want to handle this?  Three good options I can think of:
>>>> > >
>>>> > > 1) don't support migrations for sqlite (I think "no", but maybe)
>>>> > >
>>>> > > 2) Extend alembic so that op.drop_column() does the right thing
>>>> > > (more
>>>> > > open-source contributions for us, yay :) )
>>>> > >
>>>> > > 3) Add our own wrapper in savanna so that we have a drop_column()
>>>> > > method
>>>> > > that wraps copy/rename.
>>>> > >
>>>> > > Ideas, comments?
>>>> >
>>>> > Migrations should really not be run against SQLite at all -- only on
>>>> > the
>>>> > databases that would be used in production. I believe the general
>>>> > direction of the contributor community is to be consistent around
>>>> > testing of migrations and to not run migrations at all in unit tests
>>>> > (which use SQLite).
>>>> >
>>>> > Boris (cc'd) may have some more to say on this topic.
>>>> >
>>>> > Best,
>>>> > -jay
>>>> >
>>>> >
>>>> > _______________________________________________
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>>>> > OpenStack-dev at lists.openstack.org
>>>> > http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-dev
>>>>
>>>>
>>>>
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>>>
>>
>>
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