title: odoo context field. default value for popup
body: <p>I work with Odoo(v9). I have custom: form(for model 1), action and popup with form(for model 2).
  Here example main form:</p>
  
  <pre><code>&lt;record id="my_id_form" model="ir.ui.view"&gt;
      &lt;field name="name"&gt;my_name_form&lt;/field&gt;
      &lt;field name="model"&gt;my_model_one&lt;/field&gt;
      &lt;field name="arch" type="xml"&gt;
          &lt;form string="Name"&gt;
              &lt;sheet&gt;
                  &lt;group&gt;
                      &lt;field name="partner_id"/&gt;
                      &lt;!-- button which open popup with my_model_two --&gt;
                      &lt;button string="Open popup"
                              name="%(my_module.action_open_popup)d"
                              type="action"
                              class="btn-link"/&gt;
                  &lt;/group&gt;
              &lt;/sheet&gt;
          &lt;/form&gt;
      &lt;/field&gt;
  &lt;/record&gt;
  </code></pre>
  
  <p>Window action for button <em>Open popup</em>:</p>
  
  <pre><code>&lt;record id="action_open_popup" model="ir.actions.act_window"&gt;
      &lt;field name="name"&gt;action name&lt;/field&gt;
      &lt;field name="res_model"&gt;my_model_two&lt;/field&gt;
      &lt;field name="view_id" ref="model_two_form_popup"/&gt;
      &lt;!-- 
      How I can send partner_id from main form to popup? 
      I tried different ways in context field, but all in vain  
      &lt;field name="context"&gt;{'default_partner_id': ?????,}&lt;/field&gt;
      --&gt;
      &lt;field name="target"&gt;new&lt;/field&gt;
  &lt;/record&gt;
  </code></pre>
  
  <p>Example <em>my_model_one</em></p>
  
  <pre><code>class MyModelOne(models.Model):
      _name = 'my_model_one'
  
      partner_id = fields.Many2one('res.partner', string='Partner')
  </code></pre>
  
  <p>Popup form:</p>
  
  <pre><code>&lt;record id="model_two_form_popup" model="ir.ui.view"&gt;
      &lt;field name="name"&gt;Popup name&lt;/field&gt;
      &lt;field name="model"&gt;my_model_two&lt;/field&gt;
      &lt;field name="arch" type="xml"&gt;
          &lt;form string="Popup text"&gt;
              &lt;sheet&gt;
                  &lt;group&gt;
                      &lt;field name="partner_id" invisible="1"/&gt;
                  &lt;group&gt;
              &lt;/sheet&gt;
          &lt;/form&gt;
      &lt;/field&gt;
  &lt;/record&gt;
  </code></pre>
  
  <p>My question is: How I can send value <em>from field in main form</em> to <em>popup form</em>?(partner_id)</p>
  
  <p>I saw how in the code used <em>active_id</em>, string or integer values. But I have not found how to send fields values or how to register method for custom logic.
  Can someone provide a small example? Thanks in advance.</p>