parent_hash.rb

def make_parent_path(variables, prefix='')
  h = {}
  variables.each do |k,v|
    key = "#{prefix}#{k.to_s}"
    if v.is_a? Hash
      h.merge!(make_parent_path(v, "#{key}."))
    else
      h[key] = v
    end
  end
  h
end

h = {
  a: 1,
  b: {
    c: 2,
    d: 3
  }
}

make_parent_hash(h)

# {
#      "a" => 1,
#    "b.c" => 2,
#    "b.d" => 3
#}