dry_validation_and_struct.rb

require 'dry/validation'
require 'dry/types'
require 'dry/struct'

module Types
  include Dry::Types()
end

class OrderContract < Dry::Validation::Contract
  params do
    required(:discount).value(:integer)

    required(:line_items).array(:hash) do
      required(:quantity).value(:integer)
      required(:code).value(:string)
    end
  end

  rule(:line_items) do
    values[:line_items].each_with_index do |item, idx|
      key([:line_items, idx]).failure('quantity must be 1') if item[:quantity] != 1 && item[:code] == 'ABC'
    end
  end
end

class LineItem < Dry::Struct
  attribute :quantity, Types::Integer
  attribute :code, Types::String
end

class Order < Dry::Struct
  attribute :discount, Types::Integer
  attribute :line_items, Types::Array.of(LineItem)
end

contract = OrderContract.new

data = {
  discount: 4,
  line_items: [
    { quantity: 2, code: 'FOO' },
    { quantity: 3, code: 'ABC' },
    { quantity: 4, code: 'BAR' }
  ]
}

result = contract.(data)

puts result.errors.to_h.inspect
# {:line_items=>{1=>["quantity must be 1"]}}

data = {
  discount: 4,
  line_items: [
    { quantity: 2, code: 'FOO' },
    { quantity: 1, code: 'ABC' },
    { quantity: 4, code: 'BAR' }
  ]
}

result = contract.(data)

puts result.errors.to_h.inspect
# {}

order = Order.new(data)

puts order.inspect
# #<Order discount=4 line_items=[#<LineItem quantity=2 code="FOO">, #<LineItem quantity=1 code="ABC">, #<LineItem quantity=4 code="BAR">]>