dry-rules.rb

require 'dry-validation'

schema = Dry::Validation.Schema do
  required(:field_1).filled(:str?)
  required(:field_2).filled(:int?)

  rule(field_2_depends_on_field_1: [:field_1, :field_2]) do |field_1, field_2|
    field_1.eql?('Foo').then(field_2.eql?(0)) & field_1.eql?('Bar').then(field_2.eql?(1))
  end
end

puts schema.call(field_1: 'Foo', field_2: 0).success?
puts schema.call(field_1: 'Bar', field_2: 0).success?
puts schema.call(field_1: 'Foo', field_2: 1).success?
puts schema.call(field_1: 'Bar', field_2: 1).success?