ruby on rails - ActiveRecord query that groups by ID and sums rows -

my postgresql db has structure:

table orders id (string) userid (string) prodid (string) value (integer) 

this example of data:

id      userid      prodid    value 1       a@a.aaa     prod1     5 2       b@b.bbb     prod1     -1 3       a@a.aaa     prod1     -4 4       a@a.aaa     prod2     9 

i want query activerecord sums values specific userid, query (a@a.aaa) return list this:

prod1    1 prod2    9 

my first approach one, doesn't work:

orderlist = orders.select("sum(orders.amount) num_prods").where((:userid => here_user_id).group(:prodid) 

edit: rephrased feedback

order.where(userid: id).group(:prodid).sum(:value) # replace `:id` value 

this should give hash, so

{1=>10, 2=>20, 5=>20} 

the keys 1,2,5 represent product id, , values 10,20,20 represent sum values.