title: How to search product.product model with date as condition in Odoo 11
body: <p>I'd like to get a product list in Odoo 11 with several conditions by the <code>search()</code> method provided in the <code>product.product</code> model. </p>
  
  <p>This will get a list (which contains one product with code <code>12345</code> if exist)</p>
  
  <pre><code>product_object = self.env['product.product']
  product = product_object.search([('default_code', '=', '12345')])
  </code></pre>
  
  <p>But I want to filter the list to get products only if its update date is in the past compared to a given date. (NB: <code>updated_at</code> is one of my custom fields of <code>product.product</code> model, in type <code>str</code></p>
  
  <p>This won't work as expected:</p>
  
  <pre><code>product_object = self.env['product.product']
  product = product_object.search(
              ['&amp;', ('default_code', '=', '12345'), ('updated_at', '&lt;', '2017-01-10 12:01:00')])
  </code></pre>
  
  <p>The <code>product</code> variable always has nothing inside.</p>
  
  <p>How can I search the product.product model with date as condition. Must I change my field type from <code>string</code> to <code>datetime</code>?</p>