How to use PostgreSQL’s DISTINCT ON in Ecto Query

    create table(:books) do
      add :title, :string
    end

    create table(:records) do
      add :book_id, :id
      add :due_for_return, :date
    end

# lib/books.ex
defmodule Books do
  import Ecto.Query;
  alias Books.{Repo}

  def init do
    Repo.insert_all(
      "books", [
        [title: "One"],
        [title: "Two"],
        [title: "Three"],
        [title: "Four"]
      ]
    )
    Repo.insert_all(
      "records", [
        [book_id: 1, due_for_return: ~D[2017-11-01]],
        [book_id: 1, due_for_return: ~D[2017-12-01]],
        [book_id: 1, due_for_return: ~D[2018-01-01]],
        [book_id: 2, due_for_return: ~D[2018-01-02]],
        [book_id: 2, due_for_return: ~D[2018-02-02]],
        [book_id: 3, due_for_return: ~D[2018-03-03]],
      ]
    )
  end

  def query() do
    "books"
    |> join(:left, [b], r in "records", b.id == r.book_id)
    |> select([b,r], %{
         title: b.title,
         book_id: b.id,
         record_id: max(r.id)
       })
    |> group_by([b], [b.title, b.id])
    |> subquery()
    |> join(:left, [d,r],
         r in "records", d.book_id == r.book_id and d.record_id == r.id
       )
    |> select([d,r], %{
         book_id: d.book_id,
         title: d.title,
         due: r.due_for_return
       })
    |> Repo.all()
  end
end

iex(1)> Books.init()

07:15:35.715 [debug] QUERY OK db=7.9ms
INSERT INTO "books" ("title") VALUES ($1),($2),($3),($4) ["One", "Two", "Three", "Four"]
 
07:15:35.720 [debug] QUERY OK db=1.3ms
INSERT INTO "records" ("book_id","due_for_return") VALUES ($1,$2),($3,$4),($5,$6),($7,$8),($9,$10),($11,$12) [1, {2017, 11, 1}, 1, {2017, 12, 1}, 1, {2018, 1, 1}, 2, {2018, 1, 2}, 2, {2018, 2, 2}, 3, {2018, 3, 3}]
{6, nil}
iex(2)> Books.query()

07:15:43.693 [debug] QUERY OK db=3.1ms
SELECT s0."book_id", s0."title", r1."due_for_return" FROM (SELECT b0."title" AS "title", b0."id" AS "book_id", max(r1."id") AS "record_id" FROM "books" AS b0 LEFT OUTER JOIN "records" AS r1 ON b0."id" = r1."book_id" GROUP BY b0."title", b0."id") AS s0 LEFT OUTER JOIN "records" AS r1 ON (s0."book_id" = r1."book_id") AND (s0."record_id" = r1."id") []
[
  %{book_id: 1, due: {2018, 1, 1}, title: "One"},
  %{book_id: 2, due: {2018, 2, 2}, title: "Two"},
  %{book_id: 3, due: {2018, 3, 3}, title: "Three"},
  %{book_id: 4, due: nil, title: "Four"}
]
iex(3)> 

PostgreSQL GROUP BY