Looks like alembic_cfg only accepts string options. (set_main_option()). Is
there a way to pass connect object? Thanks.
Cheers,
Pan
On Wed, Dec 24, 2014 at 8:35 AM, Michael Bayer <mike...@zzzcomputing.com>
wrote:
>
>
> Pan Luo <luopa...@gmail.com> wrote:
>
> If someone new come to the project and they use db.create_all() or some
> script to create a fresh new database with declared models. There is no
> alembic_version table and he/she will run into problems when doing the
> upgrade. Is there something in alembic to support it? I have been poking
> around and came up with the following:
>
> alembic_cfg = Config("alembic.ini")
> context = MigrationContext.configure(db.engine.connect())
> current_rev = context.get_current_revision()
> context._ensure_version_table()
>
> script = ScriptDirectory.from_config(alembic_cfg)
> head_revision = script.get_current_head()
> context.stamp(script, head_revision)
>
> command.downgrade(alembic_cfg, "base")
>
> However, it works with file based sqlite db but no in memory one:
> Error: Destination base is not a valid downgrade target from current head(s)
>
> It seems the migrationcontext use different from the one I generated and
> the one used in the alembic, so the current revision was empty when I do
> the downgrade (just for. Not sure why file based sqlite works.
>
>
> A SQLite :memory: database only exists with the span of a single SQLite
> database connection. You get that above when you call
> db.engine.connect(). But then when you call command.downgrade(), you are
> not passing this connection to that command in any way; the connection is
> stuck inside your MigrationContext which isn’t present in
> command.downgrade(alembic_cfg). command.downgrade() then calls upon your
> env.py in order to get the database connection, and you probably have
> another engine.connect() in there using a different engine (and therefore
> connection pool). This makes a brand new :memory: connection that has no
> state on it.
>
> So you’d want to send an argument into the alembic_cfg in this case which
> refers to that connection from db.engine.connect(), which your env.py can
> pull out and make use of instead of connecting in some other way. This
> requires modifications to your env.py to look for this connection and use
> it, instead of connecting again. Or you can set up this connection as a
> global variable somewhere.
>
> This is a common issue so at some point I will add recipes to the
> documentation that show how to transfer a connection from the outside of
> command.xyz(), through the env.py and into the migration context.
>
>
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