title: Why a computed field is not accesible from a TransientModel in Odoo8?
body: <p><strong>ENVIRONMENT</strong></p>
  
  <p>I have created two <code>TransientModel</code> (named <code>lots.manager</code> and <code>product.lot.available</code>), with a <code>One2many</code> relationship between them (a lot manager will have several available lots).</p>
  
  <p>My purpose is to show a list of the available lots, and the user will be able to select which lots he wants to use and how much quantity from each one. So <code>product.lot.available</code> have the fields <code>lot_id</code> (to select a lot), <code>selected</code> (a <code>Boolean</code> which indicates if the lot is used or not), and <code>qty</code> (a <code>Float</code> which indicates the quantity used from each lot).</p>
  
  <p>On the other side, in <code>lots.manager</code> model, I have a computed field named <code>total_qty_selected</code>, which calculates the sum of the <code>qty</code> of all the available lots whose <code>selected</code> field is <em>True</em>.</p>
  
  <p><strong>CODE</strong></p>
  
  <pre><code>class LotsManager(models.TransientModel):
      _name = 'lots.manager'
  
      @api.multi
      @api.depends('product_lots_available', 'product_lots_available.selected',
                   'product_lots_available.qty')
      def _compute_total_qty_selected(self):
          for manager in self:
              total = 0
              for lot in manager.product_lots_available:
                  if lot.selected is True:
                      total += lot.qty
              manager.total_qty_selected = total
  
      move_id = fields.Many2one(
          comodel_name='stock.move',
          string='Stock move',
          required=True,
          select=True,
          readonly=True,
      )
      product_id = fields.Many2one(
          comodel_name='product.product',
          related='move_id.product_id',
          string='Product',
      )
      product_lots_available = fields.One2many(
          comodel_name='product.lot.available',
          inverse_name='manager_id',
          string='Available lots',
      )
      total_qty_selected = fields.Float(
          compute='_compute_total_qty_selected',
          string='Total quantity selected',
      )
  
  
  class ProductLotAvailable(models.TransientModel):
      _name = 'product.lot.available'
  
      manager_id = fields.Many2one(
          comodel_name='lots.manager',
          string='Lots Manager',
      )
      lot_id = fields.Many2one(
          comodel_name='stock.production.lot',
          string='Lot',
          readonly=True,
      )
      selected = fields.Boolean(
          string='Selected',
          default=False,
      )
      qty = fields.Float(
          string='Quantity',
          default=0.00,
      )
  
      @api.onchange('selected')
      def onchange_selected(self):
          if self.selected is True:
              _logger.info(self.manager_id.product_id.name)
              _logger.info(self.manager_id.total_qty_selected)
  </code></pre>
  
  <p><strong>PROBLEM</strong></p>
  
  <p>The computed field <code>total_qty_selected</code> is calculated well (I show it in the views and works great), but, when I try to read it from <code>product.lot.available</code> I always get 0. For example, the <code>_logger</code> lines in the <code>onchange</code> function above, show me the name of the product right, but the <code>total_qty_selected</code> returns 0, instead of in that moment I can read 2.00 in the form, or whatever value different from 0.</p>
  
  <p>I need to get the right value for doing some stuff in the onchange function.</p>
  
  <p>Can anyone help me how to manage this?</p>