Question

What’s the most concise way to determine if @hash[:key1][:key2] is defined, that does not throw an error if @hash or @hash[:key1] are nil?

defined?(@hash[:key1][:key2]) returns True if @hash[:key1] exists (it does not determine whether :key2 is defined)

Answer 1

When using ActiveSupport (Rails) or Backports, you can use try:

@hash[:key1].try(:fetch, :key2)

You could even handle @hash being nil:

@hash.try(:fetch, :key1).try(:fetch, :key2)

If you want @hash to always return a hash for a missing key:

@hash = Hash.new { |h,k| h[k] = {} }
@hash[:foo] # => {}

You could also define this recursive:

def recursive_hash
  Hash.new { |h,k| h[k] = recursive_hash }
end

@hash = recursive_hash
@hash[:foo][:bar][:blah] = 10
@hash # => {:foo => {:bar => {:blah => 10}}}

But to answer your question:

module HasNestedKey
  Hash.send(:include, self)
  def has_nested_key?(*args)
    return false unless sub = self[args.shift]
    return true if args.empty?
    sub.respond_to?(:has_nested_key?) and sub.has_nested_key?(*args)
  end
end

@hash.has_nested_key? :key1, :key2