Rails greater_than model validation against model attribute

问题:

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 starting odometer. The end odometer can also be blank because the trip may not have finished yet.

However, I can't figure out how to compare one attribute to another.

In trip.rb:

comparing against the symbol:

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

gives me the error:

ArgumentError in TripsController#index

:greater_than must be a number

comparing against the variable:

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

NameError in TripsController#index

undefined local variable or method `start_odometer' for #

回答1:

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 

回答2:

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 

回答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 

回答4:

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

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

文章来源: Rails greater_than model validation against model attribute