Hi , my problem is grouping my report value  from picking , with this condition (im using custom module to link invoice with stock picking ) 

Example :

In picking_ids field there are 2 value WH/001 and WH/002 , values come from SO1 (when i backordered the item by 2 order , they create 2 picking) 

What i want to know is , how to group by product with WH/00X as category :

Example :

WH/001

    -Item 1

    -Item 2

WH/002 

    -Item 1

inv.pp.xml

<tbody>
        <!-- TODO  Show field from model  stock.picking-->
        <!-- <tr t-foreach="o.invoice_line_ids" t-as="m"> -->
        <!-- <tr t-foreach="o.picking_ids" t-as="l"> -->
    <t t-set="product_category" t-value="[]"/>
    <t t-foreach="request.env['stock.picking'].search([])" t-as="l">
        <t t-set="product_category" t-value="product_category+[l.id]"/>
    </t>

    <t t-foreach="set(product_category)" t-as="category">

        <div><strong t-esc="category.name"/></div>
        <t t-foreach="o.picking_ids" t-as="n">
        <t t-if="category.id==n.product_id.id">

    <div><span t-field="n.product_id"/></div>

    </t>
    </t>
    </t>



</tbody>