cooperative-market/beesdoo_account/views/account_invoice.xml

<openerp>
    <data>
        <record model="ir.ui.view" id="beesdoo_account_invoice_form_view">
            <field name="name">beesdoo.account.invoice.form.view</field>
            <field name="model">account.invoice</field>
            <field name="inherit_id" ref="account.invoice_form" />
            <field name="arch" type="xml">
                <field name="date_invoice" position="attributes">
			<attribute name="required">True</attribute>
                </field>
            </field>
        </record>
    </data>
</openerp>