field auto-increment

i have a class sncp_despesa_compromisso_linha that has the following fields:
'compromisso_ano_id':fields.many2one('sncp.despesa.compromisso.ano',u'Compromisso Anual'),
'linha':fields.integer(u'Linha'),
and other fields.
i have also a class sncp_despesa_compromisso_ano that is related to this class. My idea is to put field linha of class sncp_despesa_compromisso_linha auto-increment, this is, fills automatically the field linha with number 1,2,3,....,etc.
i need to overwrite function create of class sncp_despesa_compromisso_linha? How can i do that?
i need to change xml file? How can i do that?
How can i create a ir.sequence object?
Thanks,
José