Easy Custom Rails Validators

I have a simple problem. I am working on an app that has an Event model (the kind of things that human beings attend). The model has start_time and end_time attributes. Naturally, I want to prevent users from creating time travel events where the start_time is after the end_time.

I Googled "custom rails date validation" and found a gem and a cool Validator ruby class. But rather than add yet another dependency to this app or create another file, I found a very simple solution tucked away deep in the rails guides (See section 6.2).

First off, it would be useful to know what makes an ActiveRecord object invalid. Turns out, all you need to do it add a string to the obejct's errors hash. Easy!

event.errors[:start_time] << 'No time traveling events!'
# Boom, that event is now invalid.

# You can also use the accessor method like so...
event.errors.add(:start_time, 'No time traveling events!')

Ok, good to know. Another good thing to know is that the rails validate method takes a symbol method name as a parameter that will be called upon validation. See where this is going?

# event.rb
validate :time_travel_is_illegal

So now we can simply define the method in the same file.

# event.rb
def time_travel_is_illegal
  if start_time.present? && end_time.present?
    if start_time > end_time
      errors[:base] << 'This is definitely not allowed in this universe'
    end
  end
end

Adding an error to the :base key simply means that the error is for the entire object, rather than for a specific attribute.

I like this solution because I didn't add tons of extra code and my friends can easily read what is going on.

Thanks rails for making custom validation easy.

Sincerely, James Kerr

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