code: def test_get_current_revision_multiple_heads(self):
      version_table.create(self.connection)
      context = self.make_one(
          connection=self.connection,
          opts={"version_table": "version_table"},
      )
      updater = migration.HeadMaintainer(context, ())
      updater.update_to_step(_up(None, "a", True))
      updater.update_to_step(_up(None, "b", True))
      assert_raises_message(
          CommandError,
          "Version table 'version_table' has more than one head present; "
          "please use get_current_heads()",
          context.get_current_revision,
      )
  
imports: from sqlalchemy import Column
  from sqlalchemy import inspect
  from sqlalchemy import MetaData
  from sqlalchemy import String
  from sqlalchemy import Table
  from alembic import migration
  from alembic.testing import assert_raises
  from alembic.testing import assert_raises_message
  from alembic.testing import config
  from alembic.testing import eq_
  from alembic.testing import mock
  from alembic.testing.fixtures import TestBase
  from alembic.util import CommandError
label_desc: Use image node_id 15 for calling the TestMigrationContext obj's underlying member method code with example usage: obj.test_get_current_revision_multiple_heads() without return types