deep_group.rb

require 'ostruct'

def deep_hash_with_default(level, default)
  default_value = level > 1 ? deep_hash_with_default(level - 1, default) : default
  Hash.new { |hash, key| hash[key] = default_value }  
end

def deep_group(collection, *keys)
  result = deep_hash_with_default keys.length, []
  collection.each do |item|
    # ruby 2.3
    # deep_keys = keys.map { |key| item.send key }
    # deep_item = result.dig(deep_keys)
    
    # ruby < 2.3
    deep_item = keys.inject(result) { |deep_item, key| deep_item[item.send(key)] }
    
    deep_item.push(item)
  end
  result
end


item = OpenStruct.new
item.a = 'a'
item.b = 'b'
item.c = 'c'
deep_group([item], :a, :b, :c) # => {"a"=>{"b"=>{"c"=>[#<OpenStruct a="a", b="b", c="c">]}}}