rewriter is incompatible with multi-db scheme

[sqlalchemy-bot]

Migrated issue, originally created by Michael Bayer (zzzeek)

#!python

@writer.rewrites(ops.CreateTableOp)
def create_table(context, revision, op):
    return op


def run_migrations_online():
    """Run migrations in 'online' mode.

    In this scenario we need to create an Engine
    and associate a connection with the context.

    """
    connectable = engine_from_config(
        config.get_section(config.config_ini_section),
        prefix='sqlalchemy.',
        poolclass=pool.NullPool)

    with connectable.connect() as connection:

        for schema_name in ['test_schema', 'test_schema_2']:
            conn = connection
            conn.execute("SET search_path TO %s" % schema_name)
            context.configure(
                connection=conn,
                target_metadata=target_metadata,
                upgrade_token="%s_upgrades" % schema_name,
                downgrade_token="%s_downgrades" % schema_name,
                process_revision_directives=writer,
                x_schema_name=schema_name
            )

            with context.begin_transaction():
                context.run_migrations(schema_name=schema_name)

because the upgradeops is visited multiple times, we get:

#!

File "foo/env.py", line 99, in <module>
    run_migrations_online()
  File "foo/env.py", line 94, in run_migrations_online
    context.run_migrations(schema_name=schema_name)
  File "<string>", line 8, in run_migrations
  File "/home/classic/dev/alembic/alembic/runtime/environment.py", line 836, in run_migrations
    self.get_context().run_migrations(**kw)
  File "/home/classic/dev/alembic/alembic/runtime/migration.py", line 321, in run_migrations
    for step in self._migrations_fn(heads, self):
  File "/home/classic/dev/alembic/alembic/command.py", line 156, in retrieve_migrations
    revision_context.run_autogenerate(rev, context)
  File "/home/classic/dev/alembic/alembic/autogenerate/api.py", line 415, in run_autogenerate
    self._run_environment(rev, migration_context, True)
  File "/home/classic/dev/alembic/alembic/autogenerate/api.py", line 459, in _run_environment
    hook(migration_context, rev, self.generated_revisions)
  File "/home/classic/dev/alembic/alembic/autogenerate/rewriter.py", line 102, in __call__
    self.process_revision_directives(context, revision, directives)
  File "/home/classic/dev/alembic/alembic/autogenerate/rewriter.py", line 150, in process_revision_directives
    self._traverse_list(context, revision, directives)
  File "/home/classic/dev/alembic/alembic/autogenerate/rewriter.py", line 145, in _traverse_list
    dest.extend(self._traverse_for(context, revision, directive))
  File "/home/classic/dev/alembic/alembic/autogenerate/rewriter.py", line 139, in _traverse_for
    traverser(self, context, revision, directive)
  File "/home/classic/dev/alembic/alembic/autogenerate/rewriter.py", line 110, in _traverse_script
    ret = self._traverse_for(context, revision, directive.upgrade_ops)
  File "/home/classic/dev/alembic/alembic/operations/ops.py", line 1999, in upgrade_ops
    "This MigrationScript instance has a multiple-entry "
ValueError: This MigrationScript instance has a multiple-entry list for UpgradeOps; please use the upgrade_ops_list attribute.

not sure how we can fix this because the traversal keeps happening repeatedly.