at_most: ActiveRecord limiting in Rails

tl;dr: at_most is a Ruby gem for limiting the number of AR model instances in your Rails apps

I’ve found that if I’m repeating a use-case or functionality over multiple apps, it’s probably worth extracting into a reusable gem. So in that vein, I’ve released at_most, an easy way to limit your ActiveRecord model instances.

The primary function I’ve been using this for is limiting the number of users for an app. If you’re using something like ActiveAdmin for managing your models, it’s nice to limit the number of AdminUsers for safety’s sake.

class User < ActiveRecord::Base
  at_most(5)
end

It’s really that simple. We can test this functionality by creating five users, and then attempting to create a sixth:

>> 5.times do User.create!; end

>> User.create!
=> ActiveRecord::RecordInvalid: Validation failed: The maximum number of users has been reached.

It’s important to note that this is a validation error — we can override this with our own custom error message fairly easily.

class User < ActiveRecord::Base
  at_most(5, { message: "Sorry, all our spots are full!" })
end

We can also use i18n to set an error message at a per model and global level:

en:
  activerecord:
    errors:
      models:
        users:
          at_most: "All of our spots are full :(:("
        books:
          at_most: "I can't read all of these!"
  at_most: "Something went terribly wrong."

This was a good chance for me to dig into how validations work a bit, and understand the proper way to add to them. As always, PRs and tickets are appreciated.

You can find at_most on Github here.