title: how i can pass a parent value to a one 2 many and used in an if in odoo 12
body: <p>I have this code I need if currency is MXN I can choose <code>MXN</code> and <code>USD</code> in presu_detalle, but if in presupuesto currency_id is <code>USD</code> I can't choose nothing and currency_id in presu_detalle would be <code>USD</code> by default, I tried to pass the value in the context but how I can use in python code or conditional in XML</p>
  <pre><code>name = 'presupuesto'
  
  currency_id = fields.Many2one('res.currency', 'currency')
  
  childs_ids = fields.One2many('presu_detalle', 'presu_detalle_id')
  
  name= 'presu_detalle'
  
  presu_detalle_id = fields.Many2one('Presupuesto ID')
  
  currency_id = fields.Many2one('res.currency', 'Currency')
  </code></pre>
  <p>XML form parent view</p>
  <p>field name=&quot;childs_ids&quot; nolabel=&quot;1&quot; context=&quot;{'default_currency_id':currency_id}&quot;</p>
  <p>XML Form child view</p>
  <p>field name=&quot;currency_id&quot;</p>
  <p>it passed but how i can say if the defualt value is MXN you can change value to MXN or USD but if the defualt value is USD you cant change value</p>