Check if ID exists in group of IDS in Odoo13

I want to check if my o.partner_id (which is 8) exists in this field that i made in my module. (o.invoice_line_ids.product_id.customer_specials) is a list of customers that I can add. When i display (o.invoice_line_ids.product_id.customer_specials) in a t-field it says different ids, though the o.partner_id sould be in. I guess i must check the res.partner ids in this case but I don t know how to do that. does anyone know how I can check that.

<t t-if="o.partner_id in o.invoice_line_ids.product_id.customer_specials">
    <td t-attf-class="text-left d-none d-md-table-cell">
          <span t-field="line.product_id.barcode"/>

    </td>
</t>