Ruby generate tree structure from flat array [closed]

I would like to generate tree structure from following data:

[
{"first_name" => "Test", "id" => "1", "parent_id" => ""},
{"first_name" => "Test1", "id" => "2", "parent_id" => "1"},
{"first_name" => "Test2", "id" => "3", "parent_id" => "1"},
{"first_name" => "Test3", "id" => "4", "parent_id" => "2"}
]

I would like to create ruby script to build following structure:

[
{ 
  "first_name" => "Test", 
  "id" => "1",
  "children" => [
    {
     "first_name" => "Test1", 
     "id" => "2",
     "children" => [
       {
        "first_name" => "Test3", 
        "id" => "4"
       }
     ]
    },
    {
      "first_name" => "Test2", 
      "id" => "3"
    }
}
] 

Do you have some tips in this case?