Old and new values for an attribute in ActiveRecord validation

Question

In Ruby, how would I run a validation that says that a model attribute can only be X if it used to be Y? The example would be an attribute status that takes either in-progress or complete. I'd want to say when user marks the object status, it can always be marked in-progress, but it can only be marked complete if it was first marked in-progress.

validate :status_change

def status_change
  unless self.status == "complete" && #here i want to say self.status used to be "in progress" 
    errors[:base] << "Can only mark object complete after it was first marked in progress"
  end
end

Answer 1

Please consider use ActiveModel::Dirty

It provides methods to track these changes.

before_update :status_change

def status_change
  unless status == "complete" && status_was == "in-progress" 
    errors[:base] << "Can only mark object complete after it was first marked in progress"
  end
end

Also it's not need to use self. keyword here.

Answer 2

You can use the change tracking provided by rails for this. In particular status_was returns the previous value of status. You can also use status_changed to know where the status has been changed (or else you would prevent other changes to complete models)

Personally I don't massively like the idea of validations that depend on other things than the current set of attributes. You may find that using a state machine (for example aasm) is a better way of enforcing this sort of behaviour.