Rendering cursor as collection in RoR give error undefined method 'size' for #<PostgreSQLCursor::Cursor

[ukolovda]

Hello!

I try render a collection from cursor in Ruby on Rails (it may be useful for rendering big collections).
But got an error
undefined method 'size' for #<PostgreSQLCursor::Cursor:0x00007fcc4edd4dd0

Steps to reproduce:

Create user and database:

$ psql

postgres=# create user books with password 'books';
CREATE ROLE

postgres=# create database books_development owner books;
CREATE DATABASE

Create test application:

rails new books --database postgresql
cd books

Fix development block in config/database.yml file for connect to database.

Add controller and run server:

bundle add postgresql_cursor
rails g scaffold book name:string
rails db:migrate
rails s

Testing

Open in a browser: http://localhost:3000/books
It should show book list (now empty).

Append one or several books to list.

Modify view file for use render of collection

# app/views/books/index.html.erb
<p style="color: green"><%= notice %></p>

<h1>Books</h1>

<div id="books">
<%= render partial: "books/book", collection: @books %>
</div>

<%= link_to "New book", new_book_path %>

Link http://localhost:3000/books should show book list.

Modify view index.html.erb for use postgresql_cursor enum

Append .each_instance to books collection:

<%= render partial: "books/book", collection: @books.each_instance %>

Refresh book list in browser. It show an error:

NoMethodError in Books#index

Showing books/app/views/books/index.html.erb where line #6 raised:

undefined method `size' for #<PostgreSQLCursor::Cursor:0x00007fcc4edd4dd0 @sql="SELECT \"books\".* FROM \"books\"",  ...

[ukolovda]

Errors arises in 2 places:

https://github.com/rails/rails/blob/db30dd6fe71341ac132bb1e850bd4df2596a2803/actionview/lib/action_view/renderer/collection_renderer.rb#L159

https://github.com/rails/rails/blob/db30dd6fe71341ac132bb1e850bd4df2596a2803/actionview/lib/action_view/renderer/collection_renderer.rb#L186

(Rails, last version 7.1.3.2).

It can be fixed appending to cursor singleton method size returning Float::NAN:

# app/controllers/books_controller.rb
  ...
  def index
    @books = Book.all.each_instance
    @books.define_singleton_method(:size) { Float::NAN }
  end

And removing #each_instance call in a app/views/books/index.html.erb.

Rendering works fine.

Additional effect:

• Logging row become:

Rendered collection of books/_book.html.erb [NaN times] (Duration: 24.5ms | Allocations: 11082)

• Method last? in partial rendering always false.