Context
I have several different types of books that are stored in my database. All books have some properties in common, but all book types have their own unique properties as well. Currently, I have a schema for the shared properties of books (books) and one for the non-shared properties of each type of book (type_a_books, type_b_books, etc).

  schema "books" do
    field :title, :string
    ...
    field :type, :string # "TypeABook", "TypeBBook", etc.

    has_many :publications, Publication
    many_to_many :authors, Author, join_through: "authors_books"

    has_one :type_a_book, TypeABook
    has_one :type_b_book, TypeBBook

    timestamps()
  end

  schema "type_a_books" do
    field :story, :string
    field :pictures, {:array, :string}
    belongs_to :book, Book
  end

  schema "type_b_books" do
    field :paths, {:array, :string}
    field :pictures, {:array, :string}
    field :videos, {:array, :string}
    belongs_to :book, Book
  end

I setup this model recently but I am now faced with the fact that I do not have a sufficient understanding of how to perform the needed queries. I have read the documentation of Ecto.Query, Ecto.Repo and Ecto.Schema, and learnt from several other sources.

Problem
I have not been able to figure out how to get the following data back from the database (N.B. the fields of TypeABook are joined to the fields of Book).

%{
  id: "qwerty",
  title: "A Purple Sky over Tokyo",
  ...
  type: "type_a_book",
  publications: [ #App.Publications.Publication<...> ],
  authors: [ #App.Authors.Author<...> ],
  story: "hello",
  pictures: ["url", "url", "url"],
  inserted_at: ~U[2023-02-20 13:06:54Z],
  updated_at: ~U[2023-02-20 13:06:54Z]
}

The closest I got to this result is a simple query = from d in Draft, preload: [:authors, :publications, :type_a_book]. This, however, does not add the fields of TypeABook to the fields of Book. I have also tried to use join in assoc() and join in TypeABook, but I could never get it to work fully.

Feel free to also comment on my model or the general approach I am taking.