recursive_key.rb

module RecursiveKeyCheck
  def recursive_key?(key)
    return true if key?(key)

    self.each do |_, v|
      if v.is_a?(Hash)
        return true if v.recursive_key?(key)
      end
    end

    return false
  end
end

class Hash; include RecursiveKeyCheck; end

hsh1 = { a: 1, b: 2 }
hsh2 = { b: { c: { d: { a: 1 } } }, e: 2, f: { a: 3 } }
hsh3 = { b: { c: { d: { z: 1 } } }, e: 2, f: { x: 3 } }

puts hsh1.recursive_key?(:a) # true
puts hsh2.recursive_key?(:a) # true
puts hsh3.recursive_key?(:a) # false