Never mix up greater/less than when comparing dates again

#webdev #ruby #rails

When it comes to compare dates, for some reason my brain really struggles. I mix up < and >= all the time and end up flipping them.

Is start_date greater than end_date? Or vice-versa? I get confused because I think about dates in terms of before and after not greater_than or less_than.

Usage

Luckily, Rails is here to save the day and make sure I never make this mistake again by adding before? and after? for all Date-related comparisons.

start_date = Date.new(2019, 3, 31)
end_date = Date.new(2019, 4, 1)

start_date.before? end_date
#=> true
end_date.after? start_date
#=> true

start_date = Date.new(2020, 8, 11)
end_date = Date.new(2018, 8, 11)

start_date.before? end_date
#=> false

I find myself using these methods frequently in Rails model validations when I need to ensure two dates form a valid range.

class Promotion < ApplicationRecord
  validate :valid_eligiblity_range?

  def valid_eligiblity_range?
    return unless expiration_date? && start_date?

    if !expiration_date.after?(start_date)
      errors.add(:start_date, "must be before Expiration Date")
      errors.add(:expiration_date, "must be after Start Date")
    end
  end
end