how to show specific colums in a many2many field.

this is the code i tried 

<group string="Equipment">
  <field name="equipment" string="" options="{'no_create': False}"/>
     <tree string="Equipment Details">
              <field name="name" string="Equipment Name"/>

              <field name="serial_no" string="Serial Number"/>
              


          </tree>
</group>

i get the error Field "serial_no" does not exist in model "account.asset"

but without the tree tag i get the serial_no along with other columns as always without errors. how to only show specific columns in a many2many form view