Rails greater_than model validation against model attribute

Question

I\'ve got a Trip model, which among other attributes has a start_odometer and end_odometer value. In my model, i\'d like to validate that the end odometer is larger than the st

Answer 1

It's a bit kludgy, but this seems to work:

validates_numericality_of :end_odometer, :greater_than => :start_odometer.to_i, :allow_blank => true

Answer 2

You'll probably need to write a custom validation method in your model for this...

validate :odometer_value_order

def odometer_value_order
  if self.end_odometer && (self.start_odometer > self.end_odometer)
    self.errors.add_to_base("End odometer value must be greater than start odometer value.")
  end
end

Answer 3

You can validate against a model attribute if you use a Proc.

In rails4 you would do something like this:

class Trip < ActiveRecord::Base
  validates_numericality_of :start_odometer, less_than: ->(trip) { trip.end_odometer }
  validates_numericality_of :end_odometer, greater_than: ->(trip) { trip.start_odometer }
end

Answer 4

You don't necessarily need a custom validation method for this case. In fact, it's a bit overkill when you can do it with one line. jn80842's suggestion is close, but you must wrap it in a Proc or Lambda for it to work.

validates_numericality_of :end_odometer, :greater_than => Proc.new { |r| r.start_odometer }, :allow_blank => true