Turbocharged custom Rails / ActiveRecord validations

11 Sep 2015 My code just got a lot less janky when dealing with start and end times. Before, when I wanted to validate that a user-submitted end time came after a start time, I was doing something like this:

validate :start_time_is_before_end_time
def start_time_is_before_end_time
    unless start_time.to_i < end_time.to_i
        errors.add(:end_time, 'must come after start time')
    end
end

Converting the values to epoch time and comparing isn’t terrible, but I just found a cool way to extract this out to a custom validator to make it simpler and more readable:

# app/models/model.rb
require "active_model/sequential_validator"
validates :start_time, :end_time, sequential: true

# lib/active_model/sequential_validator.rb
class SequentialValidator < ActiveModel::Validator
    def validate record
        values = options[:attributes].map do |attribute|
           record.send(attribute)
        end.compact
        if values.sort != values
            record.errors.add options[:attributes].last, "cannot be before #{options[:attributes].first}"
        end
    end
end

True, the actual validator code is not as easy to read I don’t think, but it takes some of the burden out of the model, and it also allows reusing the sequential validation in other models.

(Thanks to botandrose and the calagator project for this cool piece of code.)