Arel causing infinite loop on aggregation

Question

I have trouble with using Arel to aggregate 2 columns in the same query. When I run this, the whole server freezes for a minute, before the rails dev-server crashes. I suspect an infinite loop :).

Maybe I have misunderstood the whole concept of Arel, and I would be grateful if anybody could have a look at it.

The expected result of this query is something like this: [{:user_id => 1, :sum_account_charges => 300, :sum_paid_debts => 1000},...]

a_account_charges = Table(:account_charges)
a_paid_debts = Table(:paid_debts)
a_participants = Table(:expense_accounts_users)

account_charge_sum = a_account_charges
  .where(a_account_charges[:expense_account_id].eq(id))
  .group(a_account_charges[:user_id])
  .project(a_account_charges[:user_id], a_account_charges[:cost].sum)

paid_debts_sum = a_paid_debts
 .where(a_paid_debts[:expense_account_id].eq(id))
 .group(a_paid_debts[:from_user_id])
 .project(a_paid_debts[:from_user_id], a_paid_debts[:cost].sum)

charges = a_participants
 .where(a_participants[:expense_account_id].eq(id))
 .join(account_charge_sum)
 .on(a_participants[:user_id].eq(account_charge_sum[:user_id]))
 .join(paid_debts_sum)
 .on(a_participants[:user_id].eq(paid_debts_sum[:from_user_id]))