How to properly organize data sampling and serialization ( RoR )?

Question

Ruby on Rails

Good day.
There is a users table:
id,name,created_at,updated_at
It is required by means of AR to make a certain selection and give it to json.
There are no problems with turning it into json - @users = User.all.as_json , the problem arises in the specifics of the selection. Requires users to be grouped by creation date ( created_at ).
In other words, the sample should look something like this:

"created_at" => [
     "2015-01-01" => {
      "users" => [ { "id" => 1, "name" => "Ivan"},
              { "id" => 2, "name" => "Sergey"] }
      "2015-01-12" => {
      "users" => [ { "id" => 3, "name" => "Ivan"},
              { "id" => 4 "name" => "Sergey"] }
]

Smoking mana, alas, does not give an answer, and making a bunch of requests is expensive.
Thanks in advance for your reply.