Hash#value_at_path

hash.rb

class Hash
  # Returns the value at the specified path.
  # For example, [:foo, :bar, :baz] as a path would return the
  # value at self[:foo][:bar][:baz]. If self doesn't contain one of
  # the keys specified in path, nil is returned.
  #
  # This method is useful as it simplifies statements such as:
  #   value = h[:a][:b][:c][:d] if h[:a] && h[:a][:b] && h[:a][:b][:c]
  # to
  #   value = h.value_at_path([:a, :b, :c, :d])
  #
  # @param Array path An array of keys specifying a deep-nested path
  # @return Object the value object
  def value_at_path(path)
    raise ArgumentError unless path.is_a?(Array)
    raise ArgumentError if path.include?(nil)
    raise ArgumentError if path.empty?

    if path.size == 1
      return self[path[0]]
    else
      if self[path[0]].respond_to?(:value_at_path)
        return self[path[0]].value_at_path(path[1..-1])
      else
        return nil
      end
    end
  end
end

hash_test.rb

require 'test_helper'

class HashTest < ActiveSupport::TestCase
  def setup
    @h = { :a => { :b => {:c => 'foo'}}, :b => 'bar'}
  end

  test "value_at_path returns value if path exists" do
    assert_equal 'foo', @h.value_at_path([:a, :b, :c])
    assert_equal 'bar', @h.value_at_path([:b])
    assert_equal @h[:a], @h.value_at_path([:a])
    assert_equal @h[:a][:b], @h.value_at_path([:a, :b])
  end

  test "value_at_path returns value if path does not exist" do
    assert_nil @h.value_at_path([:a, :b, :c, :d])
    assert_nil @h.value_at_path([:c])
  end
  
  test "value_at_path raises an exception if specified path is invalid" do
    assert_raise(ArgumentError) { @h.value_at_path([]) }
    assert_raise(ArgumentError) { @h.value_at_path([nil]) }
    assert_raise(ArgumentError) { @h.value_at_path('foo') }
    assert_raise(ArgumentError) { @h.value_at_path(123) }
  end
end