Changing Business Rule Validations Over Time in Rails

Question

Several times I've been in a situation where we have a model with a business-driven validation such as:

class Order < ActiveRecord::Base
  validates_numericality_of :total, :greater_than => 5.0

  # Some more logic
end

At some point, the boss man decides that the new minimum order should be $10, so we update the validation to 10. However, this means that any existing orders with values between $5 and $10 will no longer validate, and any logic where I call order.save() will begin to fail (sometimes unpleasantly silently). I've run into this many times in a largish shipping Rails app, and haven't found a good solution yet. Some ideas:

1. Ensure that there are no pending "orders" that will be affected when the code change is rolled out
2. Add an :if => Proc.new { |o| o.created_at.nil? or o.created_at > date_new_validation_is_effective } to the new validation, but certain this quickly becomes unwieldy
3. "Validate" business rules somewhere else, such as the controllers where user-specified input is accepted, rather than as model validations. But this violates the Fat Model/Skinny Controller principle that has many proponents in Rails.

Is there a different approach for integrating this logic, or keeping a strategy like #2 manageable in the long run?

Answer 1

You could set this business logic validation to only run :on => :create. I'm assuming that you don't often edit/update the total of an order.

That would put it into effect for all orders going forward, while not affecting the validity of existing models in the system.

Answer 2

You could add a version to the order record and version specific validations.