'books' route called with redirect_to assigns variable as expected, Action :index (same page) called with render doesn't.

controller

class BooksController < ApplicationController

  def index
    @books = Book.all   #                                                  # this variable doesn't get loaded with " render "


    respond_to do |format|
      format.html
      format.yaml { render text:@books.to_yaml }
      format.json { render text:@books.to_json }
    end
  end

  def show
    @book  = Book.find(params[:id])

    respond_to do |format|
      format.html
      format.yaml { render text:@book.to_yaml }
    end
  end

  def new
    @book = Book.new
  end

  def create
    @book = Book.new(book_params)

    if @book.save
      redirect_to book_path(@book)
    else
      render :new
    end
  end

  def edit
    @book  = Book.find(params[:id])
  end

  def update
    @book  = Book.find(params[:id])
  end

  def destroy
    @book = Book.find(params[:id])
    @book.destroy
    render :index #redirect_to books                                        # here is where either render or redirect_to is called
  end

  private

  def book_params
    params.require(:book).permit(:title, :text, { author_attributes: [:name]})
  end
end