00_rom-mapper-testing.rb

require 'rom-mapper'
require 'pp'

input = {
  a: {
    b: {},
    c: {
      d: [
        {
          something: {
            key: 'value',
          },
          yay: 1
        },
        {},
        {}
      ]
    }
  }
}

class Preprocessor < ROM::Mapper
  reject_keys true

  step do
    unwrap :a do
      attribute :c
    end
  end

  step do
    unwrap :c do
      attribute :items, from: :d
    end
  end

  step do
    ungroup :items do
      attribute :something
    end
  end

  step do
    unwrap :something do
      attribute :key
    end
  end
end

preprocessor = Preprocessor.build

pp preprocessor.call([input]).first

01_output.txt

$ ruby -W0 mapper2.rb
/opt/rubies/2.2.2/lib/ruby/gems/2.2.0/gems/transproc-0.3.2/lib/transproc/hash.rb:216:in `[]': Failed to call_function map_array with [[{:something=>{:key=>"value"}}, {:something=>nil}, {:something=>nil}]] - Failed to call_function map_value with [{:something=>nil}] - Failed to call_function accept_keys with [nil] - odd number of arguments for Hash (Transproc::MalformedInputError)
	from /opt/rubies/2.2.2/lib/ruby/gems/2.2.0/gems/transproc-0.3.2/lib/transproc/hash.rb:216:in `accept_keys'
	from /opt/rubies/2.2.2/lib/ruby/gems/2.2.0/gems/transproc-0.3.2/lib/transproc/function.rb:47:in `call'
	from /opt/rubies/2.2.2/lib/ruby/gems/2.2.0/gems/transproc-0.3.2/lib/transproc/function.rb:47:in `call'
	from /opt/rubies/2.2.2/lib/ruby/gems/2.2.0/gems/transproc-0.3.2/lib/transproc/hash.rb:240:in `map_value'
	from /opt/rubies/2.2.2/lib/ruby/gems/2.2.0/gems/transproc-0.3.2/lib/transproc/function.rb:47:in `call'
	from /opt/rubies/2.2.2/lib/ruby/gems/2.2.0/gems/transproc-0.3.2/lib/transproc/function.rb:47:in `call'
	from /opt/rubies/2.2.2/lib/ruby/gems/2.2.0/gems/transproc-0.3.2/lib/transproc/composite.rb:30:in `call'
	from /opt/rubies/2.2.2/lib/ruby/gems/2.2.0/gems/transproc-0.3.2/lib/transproc/composite.rb:30:in `call'
	from /opt/rubies/2.2.2/lib/ruby/gems/2.2.0/gems/transproc-0.3.2/lib/transproc/array.rb:50:in `block in map_array!'
	from /opt/rubies/2.2.2/lib/ruby/gems/2.2.0/gems/transproc-0.3.2/lib/transproc/array.rb:50:in `map!'
	from /opt/rubies/2.2.2/lib/ruby/gems/2.2.0/gems/transproc-0.3.2/lib/transproc/array.rb:50:in `map_array!'
	from /opt/rubies/2.2.2/lib/ruby/gems/2.2.0/gems/transproc-0.3.2/lib/transproc/array.rb:41:in `map_array'
	from /opt/rubies/2.2.2/lib/ruby/gems/2.2.0/gems/transproc-0.3.2/lib/transproc/function.rb:47:in `call'
	from /opt/rubies/2.2.2/lib/ruby/gems/2.2.0/gems/transproc-0.3.2/lib/transproc/function.rb:47:in `call'
	from /opt/rubies/2.2.2/lib/ruby/gems/2.2.0/gems/rom-mapper-0.2.0/lib/rom/mapper.rb:95:in `block in call'
	from /opt/rubies/2.2.2/lib/ruby/gems/2.2.0/gems/rom-mapper-0.2.0/lib/rom/mapper.rb:95:in `each'
	from /opt/rubies/2.2.2/lib/ruby/gems/2.2.0/gems/rom-mapper-0.2.0/lib/rom/mapper.rb:95:in `reduce'
	from /opt/rubies/2.2.2/lib/ruby/gems/2.2.0/gems/rom-mapper-0.2.0/lib/rom/mapper.rb:95:in `call'
	from rom-mapper-testing.rb:53:in `<main>'