In Odoo v9 I append a button to a tree, like this

<field name="order_line_group" nolabel="1">
        <tree string="Productos" editable="bottom" create="false">
            <field name="sequence" widget="handle"/>
            <field name="product_id"/>
            <field name="name"/>
            <field name="price_unit"/>
            <field name="product_uom_qty"/>
            <field name="price_subtotal"/>
            <field name="price_total"/>
             <button name="%(module.action_name)d"
                        type="action"
                        icon="fa-th"
                        class="oe_edit_only"
                        context="{'default_product_tmpl_id': product_id}"
                />
        </tree>
 </field>

But when I click on the button, the default_product_tmpl_id it pass when the first line "product_id".... .

How do I pass the value of the line of the button clicked..