To get a many2one field of another model in your model as a drop-down list, you have to define a column or a field and a write a function for it.

Suppose, you want to get a list of invoices in a drop down list based on the customer selected or based on any other criteria , you can simple add the domain attribute to the field definition and then to retrieve them write a function.

Here is what it looks like:

1. Define a field

'partner_invoices': fields.many2one('account.invoice','CD Break Invoice',domain="[('partner_id', '=', partner_id),('invoice_line', 'ilike', 'CD Break')]",selection=_get_invoices),

Note the domain attribute here, I selected a list of all the invoices of a selected partner with penalty i.e CD Break line added to them as invoice item i.e. invoice_line.

where: partner_id is already a field defined in the same model, and 'partner_id' is the field in the account_invoice model.

2. Add the function

def _get_invoices(self, cr, uid, context=None):
        obj = self.pool.get('account.invoice')
        ids = obj.search(cr, uid,[],context=context)
        res = obj.read(cr, uid, ids, ['name', 'id'], context)
        res = [(r['id'], r['name']) for r in res]
        return res

This is how you can get a list of invoices on selecting an invoice.