如何对一个字段的所有记录的所有值求和？(Odoo 13)

from odoo import models, fields, api

class MyModel(models.Model):
    _name = 'my.model'

    field_name = fields.Float(string='Field Name')

    @api.multi
    def compute_sum(self):
        sum = 0.0
        for record in self:
            sum += record.field_name
        return sum

<record id="view_my_model_form" model="ir.ui.view">
    <field name="name">my.model.form</field>
    <field name="model">my.model</field>
    <field name="arch" type="xml">
        <form>
            <header>
                <button name="compute_sum" string="Compute Sum" type="object" class="oe_highlight"/>
            </header>
            <sheet>
                <!-- Add other fields here -->
            </sheet>
        </form>
    </field>
</record>

<record id="menu_my_model" model="ir.ui.menu">
    <field name="name">My Model</field>
    <field name="sequence" eval="10"/>
    <field name="action" ref="action_my_model"/>
</record>

<record id="action_my_model" model="ir.actions.act_window">
    <field name="name">My Model</field>
    <field name="res_model">my.model</field>
    <field name="view_mode">tree,form</field>
</record>