Improve Efficiency of Ruby Model Queries - ruby-on-rails-3

Question

Problem:
In my customer model, I want to sum up all transactions where each payments' status is successful and either the customer_id matches the id of the current instance or the current instance's parent_id. First, I was unable to get this to work at all because I was trying to do it all in one query. Now I have what I'm looking for, but I'm afraid I'm not being nearly as efficient as I could be.
Context:
The transactions table has a customer_id column
the customers table has a parent_id column.
Customer class has_many :transactions
Transactions class belongs_to :customer
Transactions class has_many :payments
Here are my models:
class Customer < ActiveRecord::Base
has_many :transactions
end
class Transaction < ActiveRecord::Base
belongs_to :customer
has_many :payments
end
class Payment < ActiveRecord::Base
has_one :transaction
end
Question:
If this is the model property I'm using currently, how can I improve its performance, if at all?
def total_spent
if customer_type == 1 # "child" customer
Transaction.joins(:payments).where('payments.status' => 3).sum(:amount, :conditions => ['customer_id = ?', id])
elsif customer_type == 2 # "parent" customer
temp_transactions = Transaction.joins(:payments).where('payments.status' => 3)
temp = temp_transactions.sum(:amount, :conditions => ['customer_id = ?', id])
Customer.find_all_by_parent_group_id(id).each do |c|
temp = temp + temp_transactions.sum(:amount, :conditions => ['customer_id = ?', c.id])
end
temp
end
end

Answer 1

Transaction.joins(:payments)
.joins(:customer)
.where(:payments => {:status => 3})
.where("customers.id = ? or customers.parent_id = ?", 5, 5)
.sum(:amount)
SELECT SUM(amount) AS sum_id
FROM "transactions"
INNER JOIN "payments" ON "payments"."transaction_id" = "transactions"."id"
INNER JOIN "customers" ON "customers"."id" = "transactions"."customer_id"
WHERE
"payments"."status" = 3
AND (customers.id = 5 or customers.parent_id = 5)