from osv import fields,osv
class first_model(osv.osv):
    _name='first.model'
    _columns={
        'second_model_ids':fields.one2many('second.model','first_model_id','Second')      
        }
first_model()

class second_model(osv.osv):
    _name='second.model'
    _columns= {
        'first_model_id':fields.many2one('first.model','First')       
        }  
second_model()

for creating a one2many field you have to create a many2one inverse relationship, thats why there in the second_model, a many2onefield is defined. And you should follow the order of definitions of both tables and you will get the result

The project_mrp module creates tasks on a project for a Sales Order with a compatible contract assigned to it.

"This module is used to automatically create tasks based on different procurements: sales order, manufacturing order, etc. It is mainly used to invoices services based on tasks automatically created by sales orders."

This is also available with version 7.0.

It can be found in the list of modules to install (ie: already comes with the software).