proc_eval_mapping.rb

require 'rom'
require 'transproc'

module Transformations
  extend Transproc::Functions

  def eval_keys(hash, param)
    hash.each_with_object(Hash[hash]) do |(key, value), copy|
      copy[key] =
        case value
        when Proc then value.call(param)
        when Hash then eval_keys(value, param)
        when Array then value.map { |v| eval_keys(v, param) }
        else value
        end
    end
  end
end

class Mapper < ROM::Mapper
  reject_keys true

  attribute :name

  attribute :foo_id, from: :foo_key do |value|
    proc { |resolver| resolver.foo_id(value) }
  end
end

class Resolver
  def self.foo_id(key)
    { 'FOO' => 123, 'BAR' => 321 }.fetch(key)
  end
end

input = [
  {
    name: 'Jane',
    foo_key: 'FOO'
  },
  {
    name: 'John',
    foo_key: 'BAR'
  }
]

mapper = Mapper.build

output = mapper.call(input)
evaled = output.map { |el| Transproc(:eval_keys, Resolver).call(el) }

puts evaled.inspect
# [{:name=>"Jane", :foo_id=>123}, {:name=>"John", :foo_id=>321}]