title: Alembic support for multiple Postgres schemas
body: <p>How can I use Alembic's <code>--autogenerate</code> to migrate <em>multiple</em> Postgres schemas that are not hard-coded in the SQL Alchemy model? (mirror question of <a href="https://stackoverflow.com/questions/9298296/sqlalchemy-support-of-postgres-schemas">SQLAlchemy support of Postgres Schemas</a>, but for Alembic).</p>
  
  <p>In particular, we use Postgres schemas to seperate different clients that share the same set of tables. Moreover, there is a schema with <em>shared</em> stuff among clients. The SQL Alchemy model has no knowledge of schemas, the schema is set at run-time using <code>session.execute("SET search_path TO client1,shared")</code>.</p>
  
  <p>The default <code>--autogenerate</code> is not helping at all, as it is detecting multiple schemas that do not exist in the model and ends up deleting the schemas and re-creating every table in the default schema.</p>
  
  <p>I would really like to use <code>--autogenerate</code> though, with the proper plumbing to set the schemas correctly. Any suggestions on if/how Alembic's API can do this?</p>