title: How to filter out only the selected records in a Many2many field from all the other records in the model in odoo13 using domain filter
body: <p>I am making a package model in which one can make packages and use it in the sales order, my model's name is sale.package.</p>
  
  <pre><code>class Package(models.Model):
      _name = 'sale.package'
      _description = 'Package table'
  
      name = fields.Char(string='Name', required=True)
      width = fields.Float(string='Width')
      height = fields.Float(string='Height')
      length = fields.Float(string='Length')
      maximum_weight = fields.Float(string='Maximum weight')
  </code></pre>
  
  <p>In order to use it in the sale order form, I inherited the sale.order model and added a Many2many field to the sale order which selects the previously made packages, I also created an Onchange function that updates an One2many field in a newly made page in the same sale order.</p>
  
  <pre><code>class SaleOrderPackage(models.Model):
      _inherit = 'sale.order'
  
      packs = fields.Many2many('sale.package', string='Package')
      package_lines = fields.One2many('sale.package.lines', 'line_name', string='Package Lines')
  
      @api.onchange('packs')
      def _onchange_packs(self):
          for rec in self:
              lines = [(5, 0, 0)]
              for line in self.packs:
                  values = {
                      'name_on_line': line.name,
                      'line_width': line.width,
                      'line_height': line.height,
                      'line_length': line.length,
                      'line_maximum_weight': line.maximum_weight,
                  }
                  lines.append((0, 0, values))
              rec.package_lines = lines
  </code></pre>
  
  <p>Up to this everything is going fine, my new requirement is to add a smart button to the same sale order and upon clicking it, a tree view of the selected packages in the sale order must show up. I have added the smart button and defined a function in the python file which returns not only the selected but also all the packages made with the model.</p>
  
  <pre><code>    def selected_packages(self):
          print(self.packs)
          return {
              'name': 'Selected Packages',
              'domain': [],
              'view_type': 'form',
              'res_model': 'sale.package',
              'view_id': False,
              'view_mode': 'tree,form',
              'type': 'ir.actions.act_window'
          }
  </code></pre>
  
  <p>As I am only a week into learning Odoo I am unable to figure out how to use the domain filter in this context, Please help.
  Thanks in advance.</p>