postgresql - Database functions mapped to ActiveRecord Models -

let's suppose have 2 database functions (pl/pgsql):

create table balances (rents decimal(12,2) default 0 not null,         expenses decimal(12,2) default 0 not null); create function account_balance(id integer)         returns balances $$         select           sum(case when t.amount > 0 t.amount else 0 end) rents,           sum(case when t.amount < 0 t.amount else 0 end) expenses         transactions t         t.account_id = $1       $$ language 'sql'; create function tenant_balance(id integer)       returns balances $$       select         sum(case when t.amount > 0 t.amount else 0 end) rents,         sum(case when t.amount < 0 t.amount else 0 end) expenses       transactions t       t.tenant_id = $1     $$ language 'sql'; 

how relate in activerecord models (in examples, account , tenant) can call account.first.balance , returns valid instance of balance? speaking of balance, how should create model?