function field , store={}...

    def _get_product_state(self,cr,uid,ids,fields,arg=None,context=None):
        res={}
        dic=dict( self.pool.get('product.product')._columns['state'].selection )
        for sol in self.browse(cr,uid,ids,):
            if sol.product_id.state in dic:
                res[sol.id]=dic[sol.product_id.state]
            else:
                res[sol.id]='N/A'
        return res
    def _get_sol_by_product(self,cr,uid,ids,context=None):
        sol_ids = self.pool.get('sale.order.line').search(cr,uid,[('product_id','in',ids),])
        return sol_ids
    _columns={
        'product_state':fields.function(_get_product_state,arg=None, type='char', string='Product State',readonly=True, store={
                    'product.product': (_get_sol_by_product, ['state',], 20),   
                    'sale.order.line': (lambda self, cr, uid, ids, c={}: ids, ['product_id'], 20),                                 
                     }), 

store={ 'model.name': (  function_return_ids,  fieds_name_list, 20 ),}

when model.name's   field(include by fields_name_list) change, the  function_return_ids will return the ids ,

those ids will re-counter the function field and store it.