How to compare DateTime in custom validations?

I'm trying to write a custom validation...people enter in a start and end time into a site and I'm trying to write a validation to check that the end comes after the start. I've tried to convert it to Unix time, but then I get a noMethodError. I would like to pass the symbols for the start time and end time into this method and then compare the two.

  def validates_end_is_after_start(*attr_names)

    start_hour = attr_names[0].to_i
    end_hour = attr_names[1].to_i

The p开发者_高级运维roblem is that this ends up with a noMethodError for symbol. But that symbol points to a DateTime object (I think), how do I access that object? I think that there's something I don't understand about symbols in general.

You could use the validate method which would have access to type-casted dates

class Something < ActiveRecord::Base

  validate :valid_dates

  def valid_dates
    if start_time >= end_time
      self.errors.add :start_time, ' has to be after end time'
    end
  end

end

Think of symbols just like strings, but with a lot less of functionality and behavior as objects. You should change your code to following:

def validates_end_is_after_start(*attr_names)
  start_hour = self.send(attr_names[0].to_sym).to_i
  end_hour = self.send(attr_names[1].to_sym).to_i
  # ... more code here
end

You should also checkout the awesome gem validates_timeliness, which is made to handle validation of date, time and datetime.

Since Ruby on Rails 7.0 there is no need anymore for a custom validation in this case because Rails 7.0 supports validates_comparison_of like this

validates :start_time, comparison: { less_than: :end_date }