comment_view.ex

defmodule MyApp.CommentView do
  use MyApp.Web, :view
  @attributes ~W(id name inserted_at)
  
  def render("index.json", %{data: comments}) do
    for comment <- comments, do: render("show.json", %{data: comment})
  end
  
  def render("show.json", %{data: comment}) do
    comment
    |> Map.take(@attributes)
    |> put_assoc(:user, comment.user)
  end
end

mymodel_view.ex

defmodule MyApp.MyModelView do
  use MyApp.Web, :view
  @attributes ~W(id body title inserted_at)
  
  def render("show.json", %{data: my_model})
    my_model
    |> Map.take(@attributes)
    |> Map.update!(:title, &String.capitalize)
    |> put_assoc(:user, my_model.user)
    # Or you could do
    # |> put_assoc(:user, my_model.user, :lite)
    |> put_assoc(:comments, my_model.comments)
  end
end

user_view.ex

defmodule MyApp.UserView do
  use MyApp.Web, :view
  @attributes ~W(id name inserted_at)
  
  def render("show.json", %{data: user})    
    render("show_lite.json", %{data: user})
    |> Map.put(:image, ImageView.render("show.json", %{data: user.image})
  end
  
  def render("show_lite.json", %{data: user})    
    user
    |> Map.take(@attributes)
  end
end

web.ex

defmodule MyApp.Web do
  def view do
    quote do
      # Default code. 
      
      # You would probably extract these functions to a module.
      def put_assoc(map, :user, user) do
        Map.put(map, :user, UserView.render("show.json", %{data: user})
      end
      
      # Need to customize more?
      def put_assoc(map, :user, user, :lite) do
        Map.put(map, :user, UserView.render("show_lite.json", %{data: user})
      end
      
      def put_assoc(map, :comments, comments) do
        Map.put(map, :comments, CommentView.render("index.json", %{data: comments})
      end
    end
  end
end