title: Odoo - Dropdown search in related model
body: <p>I've got the following model:<br/>
  groups - partners<br/>
  groups - vehicles<br/>
  <br/>
  partners &lt;- vehicles</p>
  
  <p>Vehicles are related to partners (each partner can have multiple buses) via the x_partner_id field.</p>
  
  <p>I would like to assign a partner and a bus to each group with the possibility to chose a bus from a different partner. The form looks like this:</p>
  
  <pre class="lang-xml prettyprint-override"><code>&lt;form&gt;
      &lt;sheet string="Groups"&gt;
          &lt;group col="4"&gt;
              &lt;field name="x_date_start" modifiers="{}"/&gt;
              &lt;field name="x_date_stop" modifiers="{}"/&gt;
              &lt;field name="x_partner_id" can_create="true" can_write="true" modifiers="{}"/&gt;
              &lt;field name="x_name" modifiers="{}"/&gt;
              &lt;field name="x_vehicle_id" can_create="true" can_write="true" modifiers="{}"/&gt;
              &lt;field name="x_groupcode" modifiers="{}"/&gt;
              &lt;newline/&gt;
              &lt;field colspan="4" name="x_stops_id" modifiers="{}"/&gt;
              &lt;newline/&gt;
              &lt;separator/&gt;
          &lt;/group&gt;
      &lt;/sheet&gt;
  &lt;/form&gt;
  </code></pre>
  
  <p>Right now, if I type any property in the x_vehicle_id field on the form, I can find the bus I need. Can someone give me a hint, how to modify this field in the form to extend this search to other models related to the vehicles model? So, if I enter a partner name, I want to see only the buses from that partner.</p>