validation-rules.rb

require 'bundler/inline'

gemfile(true) do
  source 'https://rubygems.org'

  gem 'dry-validation'
end

schema = Dry::Validation.Schema do
  optional(:y).filled(:int?)
  optional(:z).filled(:odd?)
  optional(:x).filled(:int?)
    .when(eql?: 1) {
      required(:y).filled(:odd?)
    }
end

p schema.({})
p schema.({x: 1})
p schema.({x: 2})
p schema.({x: 1, y: 1})
p schema.({x: 2, y: 1})
p schema.({x: 1, y: 2})
p schema.({x: 2, y: 2})
p schema.({z: 1})
p schema.({z: 2})

#<Dry::Validation::Result output={} errors={}>
#<Dry::Validation::Result output={:x=>1} errors={:y=>["is missing"]}>
#<Dry::Validation::Result output={:x=>2} errors={}>
#<Dry::Validation::Result output={:x=>1, :y=>1} errors={}>
#<Dry::Validation::Result output={:x=>2, :y=>1} errors={}>
#<Dry::Validation::Result output={:x=>1, :y=>2} errors={:y=>["must be odd"]}>
#<Dry::Validation::Result output={:x=>2, :y=>2} errors={}>
#<Dry::Validation::Result output={:z=>1} errors={}>
#<Dry::Validation::Result output={:z=>2} errors={:z=>["must be odd"]}>