How to update the value of field of from view when update the field of tree view?

I want to change the value of "total_location_qty" when I update the value of "location_qty".

This is my obj

class Demo(models.TransientModel):
    _name = 'demo.parent'
    demo_lines_ids = fields.One2many('demo.lines', 'demo_id', 'demo Items')
    total_location_qty = fields.Float(string='Total of location qty')

class DemoLines(models.TransientModel):
    _name = "demo.lines"
    demo_id = fields.Many2one('demo.parent', 'Demo')
    location_id = fields.Many2one('stock.location', 'Stock Location', readonly=True)
    location_qty = fields.Float(string='Location Qty')

    @api.onchange('location_qty')
    def on_change_location_qty(self):
        self.demo_id.total_location_qty = self.location_qty

This is my view

<record id="view_demo_wizard" model="ir.ui.view">
	<field name="name">Demo</field>
        <field name="model">demo.parent</field>
        <field name="arch" type="xml">
            <form string="Demo Parent">
	    <group string="Demo">
		<group colspan="4">
                    <field name="total_location_qty" readonly="1"/>
                </group>
		<field name="demo_lines_ids" nolabel="1"   >
   		    <tree string="Demo Lines" editable="bottom" create="false" delete="false" >
			<field name="location_qty"/>
		    </tree>
                 </field>
             </group>
	    </form>
        </field>
</record>