Log in as Manager::

    >>> browser.login(TEST_USER_NAME, TEST_USER_PASSWORD)

Go to edit order container::

    >>> browser.open('/order-container/edit')
    >>> browser.getControl(name="form.widgets.next_order_id").value = '2'
    >>> browser.getControl(name="form.buttons.save").click()
    >>> print browser.bycss('.documentDescription')
    --- 1 ---
    <div class="documentDescription">The next order ID: 2</div>

Go to edit order container for the validation::

    >>> browser.open('/order-container/edit')
    >>> browser.getControl(name="form.widgets.next_order_id").value = '1'
    >>> browser.getControl(name="form.buttons.save").click()

    >>> browser.url
    'http://nohost/plone/order-container/@@edit'

    >>> print browser.bycss('.error')
    --- 1 ---
    <dl class="portalMessage error"><dt>
    Error
    </dt>
    <dd>There were some errors.</dd>
    </dl>
    --- 2 ---
    <div ...>
    <label for="form-widgets-next_order_id" class="horizontal">
    Next Order ID
    <span class="required horizontal" title="Required">&#160;</span>
    </label>
    <div class="fieldErrorBox">
    <div class="error">The order ID is already in use.</div>
    </div>
    ...