class Person < ActiveRecord::Base
has_many :transactions
def balance
transactions.sum(:amount)
end
end
class Transaction < ActiveRecord::Base
belongs_to :person
end
Given the above, what is a good way to efficiently and DRYly find
people by balance?
Finding all records, then using the association is very slow.
Person.find(:all).select { p.balance > 100 }
A subselect is much faster, but completely duplicates the logic.
Person.find(:all,
:select => “people.*, (SELECT SUM(amount) FROM transactions WHERE
people.id=transactions.person_id) AS balance”,
:conditions => “(SELECT SUM(amount) FROM transactions WHERE
people.id=transactions.person_id) > 100”
Especially as the calculations get more complicated this starts to
become a real problem.
Thanks.
Jack Christensen
