Hi guys,

I have two different many2one relations. This are the fields that I define in my model:

     _columns = {
        'product_id': fields.many2one('product.product','Product'),
        'car_id': fields.many2one('fleet.vehicle', 'Car'),
        'analytic_account_id': fields.many2one('account.analytic.account', 'Analytic Account'),
        'analytic_car_account_id': fields.many2one('account.analytic.account', 'Analytic Account 2'),
        'name': fields.text('Description', required=True),
        'quantity': fields.float('Quantity', required=True),
        'uom_id': fields.many2one('product.uom', 'Unit of Measure',required=True),
        'price_unit': fields.float('Unit Price', required=True),
        'price_subtotal': fields.function(_amount_line, string='Sub Total', type="float",digits_compute= dp.get_precision('Account')),
        'car_license_plate': fields.text('Nummerplaat', required=True),
        'car_rent_price': fields.float('Prijs', required=True),
        'car_number_of_units': fields.integer('Aantal'),
    }

As you can see I have multiple fields that are required, since they really need to be filled in. Most of these fields are used in the many2one product_id and four of these fields are for the many2one car_id.
As you can see in the XML view:

 <div attrs="{'invisible': [('recurring_invoices','=',False)]}">
                        <field name="recurring_invoice_line_ids" >
                            <tree string="Account Analytic Lines" editable="bottom">
                                <field name="product_id" on_change="product_id_change(product_id, uom_id, quantity, False, parent.partner_id, False, parent.pricelist_id, parent.company_id)"/>
                                <field name="name"  attrs="{'required':[('product_id', '=', True)]}"/>
                                <field name="quantity" attrs="{'required':[('product_id', '=', True)]}"/>
                                <field name="uom_id" attrs="{'required':[('product_id', '=', True)]}"/>
                                <field name="price_unit" attrs="{'required':[('product_id', '=', True)]}"/>
                                <field name="price_subtotal"/>
                            </tree>
                        </field>
                        <field name="recurring_invoice_car_line_ids">
				<tree string="Account Analytic Lines" editable="bottom">
					<field name="car_id" on_change="car_id_change(car_id, car_license_plate, car_rent_price)"/>
					<field name="car_license_plate" attrs="{'required':[('car_id', '=', True)]}"/>
					<field name="car_rent_price" attrs="{'required':[('car_id', '=', True)]}"/>
					<field name="car_number_of_units" attrs="{'required':[('car_id', '=', True)]}"/>
				</tree>
			</field> 
</div>

My question is now how can I guarantee that the fields must be filled in, depending on the requirements?
All the samples:
A user wants to add one product to the many2one product_id but nothing to car_id. 
=> Only the fields for the many2one product are required (as specified in the model).
A user wants to add one car to the many2one car_id but nothing to product_id
=> Only the fields for the many2one car_id are required (as specified in the model).
A user wants to add both product(s) and car(s) to both many2one relations  (as specified in the model).
=> All fields on the many2one product_id should be filled in and all fields on the many2one car_id should be filled in.

How exactly should I do this? I know I need domains but I don't know how to only force the user to fill in fields when something in the many2one is filled in..
I've tried a few ways but I keep getting this dialog