Hi, I'm trying to put a searchable field in POS Order Analysis to filter by TAX. So far I created 'tax_id' field of type many2many

class pos_order_report(osv.osv):
_name = "report.pos.order"
_description = "Point of Sale Orders Statistics"
_auto = False
_columns = {
   ...
   'tax_id': fields.many2many('account.tax','product_taxes_rel','prod_id','tax_id','Product Tax',readonly=True ),
   ...

and added it in the View:

        ...
        <record id="view_report_pos_order_tree" model="ir.ui.view">
        <field name="name">report.pos.order.tree</field>
        <field name="model">report.pos.order</field>
        <field name="arch" type="xml">
            <tree string="Point of Sale Analysis" create="false">
               ...
                <field name="tax_id" invisible="1"/>
                ...


        <record id="view_report_pos_order_search" model="ir.ui.view">
        <field name="name">report.pos.order.search</field>
        <field name="model">report.pos.order</field>
        <field name="arch" type="xml">
            <search string="Point of Sale Analysis">
                ...
                <field name="tax_id"/>
               ...

It let me search by Product Tax but it shows wrong results. What is the right approach for this? I'll appreciate any help. Thanks