Add a custom field to existing model's Group By... on the Search for Sales Analysis Report in Odoo 8

sale_report.py

from openerp import tools
from openerp.osv import fields, osv

class SaleReport(osv.osv):
    _inherit = 'sale.report'

    _columns = {
        'mage_store': fields.many2one('mage.store.view', 'Magento Store', readonly=True),
    }

    def _select(self):
        return super(SaleReport,self)._select() + ', s.mage_store as mage_store'

    def _group_by(self):
        return super(SaleReport,self)._group_by() + ', s.mage_store'

sale_report.xml

<?xml version="1.0"?>
<openerp>
    <data>

        <record id="view_mage_store_sale_report_search" model="ir.ui.view">
            <field name="name">Group By Magento Store on Sale Report Search</field>
            <field name="model">sale.report</field>
            <field name="inherit_id" ref="sale.view_order_product_search" />
            <field name="arch" type="xml">
                <filter string="Company" position="after">
                    <filter string="Magento Store" context="{'group_by': 'mage_store'}"/>
                </filter>
            </field>
        </record>

    </data>
</openerp>