Odoo: How to count in Tree View when group_by?

Question

I'm trying to count plan_payment_number in tree view when group_by, but SUM by default in Odoo v8. I need only count plan_payment_number.

This is what I have now:

And this is what I need:

I tried with count = "true". My code on the tree view:

<record id="view_plan_car_tree" model="ir.ui.view">
    <field name="name">plan.car.tree</field>
    <field name="model">plan.car</field>
    <field name="arch" type="xml">
    <tree colors="blue:state == 'draft';black:state == 
    'done';gray:state == 'cancel'" string="Payments">
        <field name="number"/>
        <field name="plan_payment_number" />
        <field name="date_to"/>
        <field name="name"/>
        <field name="state"/>
        <field name="contract_id"/>
        <field name="partner_id"/>
        <field name="initial_quota"/>
        <field name="capital_quota"/>
        <field name="administrative_quota"/>
        <field name="pay_type"/>
        <field name="company_id" groups="base.group_multi_company" 
        widget="selection"/>
        <field name="plan_run_id" invisible="1"/>
    </tree>
    </field>
</record>

Answer 1

My solution for this problem is:

class plan_car(models.Model):
    _inherit = 'plan.car'
def read_group(self, cr, uid, domain, fields, groupby, offset=0, limit=None, 
context=None, orderby=False, lazy=True):
res = super(plan_car, self).read_group(cr, uid, domain, fields, groupby, 
offset, limit=limit, context=context, orderby=orderby, lazy=lazy)
if 'plan_payment_number' in fields:
    for line in res:
        if '__domain' in line:
            lines = self.search(cr, uid, line['__domain'], context=context)
            pending_value = 0
                for current_account in self.browse(cr, uid, lines, 
                context=context):
                    pending_value += 1
                    line['plan_payment_number'] = pending_value
return res