Rails validation method comparing two fields?

Question

My model has two fields that i want to compare to each other as part of the validation. I want to be sure end_time is after start_time. I have written a validation method to compare them, but I must be doing something wrong, because the values are always nil. Can someone help?

class LogEntry < ActiveRecord::Base
  validates :start_time, :presence => { :message => "must be a valid date/time" }
  validates :end_time, :presence => {:message => "must be a valid date/time"}
  validate :start_must_be_before_end_time

  def start_must_be_before_end_time
    errors.add(:start_time, "must be before end time") unless
       start_time > end_time
  end 
end

gets the error

undefined method `>' for nil:NilClass

So, start_time and/or end_time are nil. I thought I was following the many examples I found, but apparently not. What am I missing?

Thanks.

Answer 1

My best guess is you need your method to look like this:

private

def start_must_be_before_end_time
    errors.add(:start_time, "must be before end time") unless
        start_time < end_time
end 

(Also, notice the < rather than > (or change to if and >=)

If this doesn't work then you should also check that start_time and end_time are being defined correctly in the controller as there can be funny things happen if the time is created across more than one form element.

With Rails 7 ComparisonValidator

Rails 7 has added the ComparisonValidator which allows you to use the convenience method validates_comparison_of like so:

class LogEntry < ActiveRecord::Base
  validates_comparison_of :start_time, less_than: :end_time
  # OR
  validates_comparison_of :end_time, greater_than: :start_time
end

Find out more here: https://api.rubyonrails.org/classes/ActiveModel/Validations/HelperMethods.html#method-i-validates_comparison_of