How do I define helper methods to be used across rules?

Support dry-validation
1

I have rules with repeated logic in them that I would like to extract to prevent duplication. Here, I’m using values[:age] < 21 as an example of the repeated logic:

# frozen_string_literal: true

require 'dry-validation'

class MyContract < Dry::Validation::Contract
  params do
    required(:age).value(:integer)
    optional(:rent_car).value(:bool)
    optional(:rent_boat).value(:bool)
  end

  rule(:rent_car, :age) do
    key.failure("Not old enough to rent a car") if values[:age] < 21
  end

  rule(:rent_boat, :age) do
    key.failure("Not old enough to rent a boat") if values[:age] < 21
  end
end

validation = MyContract.new.call(age: 10, rent_car: true, rent_boat: true)
p validation.errors[:rent_car]
p validation.errors[:rent_boat]

My naive attempt to define a method fails:

class MyContract < Dry::Validation::Contract
  params do
    required(:age).value(:integer)
    optional(:rent_car).value(:bool)
    optional(:rent_boat).value(:bool)
  end

  rule(:rent_car, :age) do
    key.failure("Not old enough to rent a car") unless old_enough?
  end

  rule(:rent_boat, :age) do
    key.failure("Not old enough to rent a boat") unless old_enough?
  end

  def old_enough?
    values[:age] >= 21
  end
end

Traceback (most recent call last):
	12: from repro.rb:25:in `<main>'
	11: from /.../gems/dry-validation-1.2.1/lib/dry/validation/contract.rb:94:in `call'
	10: from /.../gems/dry-validation-1.2.1/lib/dry/validation/result.rb:25:in `new'
	 9: from /.../gems/dry-validation-1.2.1/lib/dry/validation/contract.rb:95:in `block in call'
	 8: from /.../gems/dry-validation-1.2.1/lib/dry/validation/contract.rb:95:in `each'
	 7: from /.../gems/dry-validation-1.2.1/lib/dry/validation/contract.rb:98:in `block (2 levels) in call'
	 6: from /.../gems/dry-validation-1.2.1/lib/dry/validation/rule.rb:38:in `call'
	 5: from /.../gems/dry-validation-1.2.1/lib/dry/validation/rule.rb:38:in `new'
	 4: from /.../gems/dry-validation-1.2.1/lib/dry/validation/evaluator.rb:73:in `initialize'
	 3: from /.../gems/dry-validation-1.2.1/lib/dry/validation/evaluator.rb:73:in `instance_exec'
	 2: from repro.rb:13:in `block in <class:MyContract>'
	 1: from /.../gems/dry-validation-1.2.1/lib/dry/validation/evaluator.rb:190:in `method_missing'
repro.rb:21:in `old_enough?': undefined local variable or method `values' for #<MyContract:0x00007fa3799928d8> (NameError)

What is the appropriate pattern to avoid repeating logic like this?

2

What do you think about:

class MyContract < Dry::Validation::Contract
  params do
    required(:age).value(:integer)
    optional(:vehicle).value(:bool)
  end

  rule(:vehicle, :age) do
    key.failure("Not old enough to rent a vehicle") if values[:age] < 21
  end
end
3

You could do this:

require 'dry-validation'

class MyContract < Dry::Validation::Contract
  register_macro(:old_enough?) do
    type = keys.last

    if values[type] && value < 21
      name = { rent_car: "a car", rent_boat: "a boat" }[type]
      key.failure("Not old enough to rent #{name}")
    end
  end
  
  params do
    required(:age).value(:integer)
    optional(:rent_car).value(:bool)
    optional(:rent_boat).value(:bool)
  end

  rule(:age, :rent_car).validate(:old_enough?)
  rule(:age, :rent_boat).validate(:old_enough?)
end

my_contract = MyContract.new

puts my_contract.(age: 12, rent_car: true, rent_boat: true).errors.inspect
#<Dry::Validation::MessageSet messages=[#<Dry::Validation::Message text="Not old enough to rent a car" path=[:age] meta={}>, #<Dry::Validation::Message text="Not old enough to rent a boat" path=[:age] meta={}>] options={}>

puts my_contract.(age: 12, rent_car: true, rent_boat: false).errors.inspect
#<Dry::Validation::MessageSet messages=[#<Dry::Validation::Message text="Not old enough to rent a car" path=[:age] meta={}>] options={}>

puts my_contract.(age: 12, rent_car: false, rent_boat: true).errors.inspect
#<Dry::Validation::MessageSet messages=[#<Dry::Validation::Message text="Not old enough to rent a boat" path=[:age] meta={}>] options={}>

puts my_contract.(age: 21, rent_car: true, rent_boat: true).errors.inspect
#<Dry::Validation::MessageSet messages=[] options={}>

puts my_contract.(age: 12, rent_car: false, rent_boat: false).errors.inspect
#<Dry::Validation::MessageSet messages=[] options={}>

To make it nicer, I would use localized messages and use failure identifier rather than building up that string message.

6

For anyone else reading this post, it is (now) possible to include methods and modules inside the contract class that can be accessed in the rule. I have been trying to work out how to have the rules short circuit (ie. not continue for a parameter once the first rule has failed) and this is the only vaguely elegant way I’ve worked out how to do it so far.

require "bundler/inline"

gemfile do
  source "https://rubygems.org"
  gem "dry-validation", "1.10"
  gem "pry-byebug"
end

module ValidationHelpers
  extend self

  def validate_phone_number(value, key)
    if !value.start_with?("04")
      key.failure("Phone number must start with 04")
    end
  end

  def validate_not_blank(value, key)
    if value.strip.size == 0
      key.failure("must contain some chars")
    end
  end
end

Dry::Validation.register_macro(:not_blank) do
  ValidationHelpers.validate_not_blank(value, key)
end

class PersonContract < Dry::Validation::Contract
  include ValidationHelpers

  json do
    required(:phone_number).filled(:string)
  end

  rule(:phone_number).validate(:not_blank)

  rule(:phone_number) do
    validate_phone_number(value, key) unless rule_error?(:phone_number)
  end
end

puts PersonContract.new.call({ phone_number: " ", foo: nil}).errors.to_hash
# {:phone_number=>["must contain some chars"]}

puts PersonContract.new.call({ phone_number: "1", foo: nil}).errors.to_hash
# {:phone_number=>["Phone number must start with 04"]}