how can i get the som of coef for every classe

class gestion_classe(osv.osv):

_name = "gestion.classe"

_rec_name = "nomc"

_description = "Table des classes"

_columns = {

'nomc': fields.char('Nom de la classe',required=True),

'numg_ids': fields.one2many('gestion.groupe','classe_id','Groupes'),

'numm_ids': fields.one2many('gestion.matiere','classe1_id','Matieres'),

'somme_coef':

}

class gestion_matiere(osv.osv):

_name = "gestion.matiere"

_rec_name= "libelle"

_description = "Table des matieres"

_order = "numm asc"

_columns = {

'numm': fields.integer('Code matiére',required=True),

'libelle': fields.char('Libelle'),

'classe1_id': fields.many2one('gestion.classe','Classe',ondelete='cascade'),

'coef' : fields.integer('Coefficient matière'),

'prof_ids': fields.one2many('gestion.prof','matiere_id','Enseignants'),

'eleve_ids' :fields.one2many('gestion.note.matiere','asso1_id','Matiere'),

}