Hello, 

In the new version , odoo v9, I've seen several relations like:

product.product with product.template

res.user with res.partner

account.payment with  account.abstract.payment

I still don't understand this , I thought that if I added a field in account.abstract.payment , that new field would be present in the account.payment , but it doesn't...

In core code we have this: 

class account_abstract_payment(models.AbstractModel):
    _name = "account.abstract.payment"
    _description = "Contains the logic shared between models which allows to register payments"
...
class account_register_payments(models.TransientModel):
    _name = "account.register.payments"
    _inherit = 'account.abstract.payment'
    _description = "Register payments on multiple invoices"
....
class account_payment(models.Model):
    _name = "account.payment"
    _inherit = 'account.abstract.payment'
    _description = "Payments"
    _order = "payment_date desc, name desc"
....

So, I thought that if I do this :

class account_abstract_payment(models.AbstractModel):
    _inherit = "account.abstract.payment"  
    payment_line_ids = fields.One2many('account.payment.lines', 'payment_id', string='Payment Lines',) 

I thougth that I was adding the field to account_register_payments and account_payment. 

But when I implement the view for account_payment:

<record id="view_account_payment_form_saft" model="ir.ui.view">
            <field name="name">account.payment.form.saft</field>
            <field name="model">account.payment</field>
            <field name="inherit_id" ref="account.view_account_payment_form"/>
            <field name="arch" type="xml">
                <sheet position="inside">
                    <group name="payment_lines" string="Payment Lines">
                        <field name="payment_line_ids" attrs="{'readonly': [('state', '!=', 'draft')]}"/>
                    </group>
                </sheet>   
            </field>
        </record>

I get this error :

Error details:
Field `payment_line_ids` does not exist

Is there a documentation or something for me to understand how this work? 

Thanks