i have a many2one field and i want that if certain value is selected a field must get invisible . There are many questions like that on the forum but nothing working for me. Please help

here is the code

py

class MaintNewCust(models.Model):
    _inherit = 'maintenance.request'

    number = fields.Char(string='Serial Number')
    complaint_type = fields.Many2one('my.model',string='Complaint Type')
    emr = fields.Selection([
        ('ws', 'Water Supply'),
        ('sui', 'Sui Gas'),('electric', 'Electric'),('SA', 'Split AC')], string='E & M', copy=False)
    bbr = fields.Selection([
        ('fw', 'Finishing Work'),
        ('sf', 'Sanitary Fitting'), ('ss', 'Sewarage System'), ('fp', 'Footpaths'), ('rd', 'Roads'), ('ww', 'Wood Works')], string='B & R', copy=False)
    used = fields.Boolean(string='Boolean')

    @api.onchange('complaint_type')
    def _onchange_type(self):
        if self.complaint_type == "Buildings and roads":
            self.used = True
        else:
            self.used = False

xml 

    <xpath expr="//field[@name='employee_id']" position="after">
        <field name="complaint_type"/>
    </xpath>
<xpath expr="//field[@name='maintenance_type']" position="after">
        <field name="emr" attrs="{'invisible': [('used','==',False)]}"/>
    </xpath>
  <xpath expr="//field[@name='maintenance_type']" position="after">
        <field name="bbr" attrs="{'invisible':[('used', '==', True)]}"/>
    </xpath>