When saving computed values in the database in your rails app, you must be aware that is possible to find unexpected errors in the result thanks to race conditions.
I have already shared an exercise to help you get more sensitivity about when an implementation can save a wrong value thanks to race conditions.
Here I want to share one tip you can try to avoid race conditions when saving a computed value.
To talk about something concrete I will use the “account balance” as an example, but you can use this approach for different types of calculations.
Imagine that you have an
Account record that has many
entries, and you want to update the
balance each time an
Entry is created. The
balance is the sum of the
amount of each
Imagine each account will need to create many entries concurrently, maybe on different background jobs or different requests. So if you want to calculate the balance and save it just after an entry is created, you could have problems with race conditions.
If your app is really concurrent and you can’t show posible off value, maybe you can pick a “safe previous date” like “yesterday” or “an hour ago” and present the value for that date.
You can run a daily rake task to save the balance with something like this:
class Account < ApplicationRecord has_many :entries def balance_at(datetime) entries.where(created_at: ..datetime).sum(:amount) end def update_safe_balance! self.safe_balance_time = 1.day.ago.end_of_day self.safe_balance = balance_at(safe_balance_time) save! end def update_safe_balance_later Account::UpdateSafeBalanceJob.perform_later(self) end end class Entry < ApplicationRecord belongs_to :account, touch: true end class UpdateSafeBalanceJob < ApplicationJob def perform(account) account.update_safe_balance! end end # on a daily task Account.find_each(&:update_safe_balance_later)
And then you can use it in your views indicating the time of the calculation:
<p> Balance: <%= number_to_currency account.safe_balance %> <small>Updated <%= account.safe_balance_time %></small> </p>
As far as I understand, yes! We are avoiding race conditions because we are calculating the balance for a “safe” point in time. What does “safe” means will depend on your use case.
No, because you are not really showing the “current value”, and you will need to make sure that your users understand that this is a calculation for a previous point in time, and that can be confusing.
I learned this tip from recycledcoder on reddit.
If you have experience with other problems with this solution, please leave a comment =)
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