Simple postgres cursor

Cursor.rb

# Use as
Cursor.new(Model.includes(foos: [:bar], :foo_bars).where(id < 1000000)).find_each do |model|
  # do something
end

# Inspired by https://github.com/afair/postgresql_cursor
class Cursor
  class NoopObserver
    def close_failed(e)
    end

    def fetching(size)
    end
  end

  def initialize(query, fetch_size: 1000, observer: NoopObserver.new)
    @query = query
    @cursor_name = "my-cursor-#{SecureRandom.uuid}".gsub('-', '')
    @stmt_name = "my-stmt-#{SecureRandom.uuid}".gsub('-', '')
    @count = 0
    @fetch_size = fetch_size
    @observer = observer
  end

  def find_each
    ActiveRecord::Base.transaction do
      begin
        open

        while (result = fetch) do
          records = result.map { |row| @query.klass.new(row.symbolize_keys) }
          break if records.size == 0

          load_associations!(records)

          records.each do |record|
            @count += 1
            record.readonly!
            yield(record)
          end
        end
      ensure
        close
      end
    end
  end


  private
  def load_associations!(rows)
    preload = @query.includes_values
    preloader = ActiveRecord::Associations::Preloader.new
    preload.each do |associations|
      preloader.preload rows, associations
    end
  end

  def open
    @conn = ActiveRecord::Base.connection.raw_connection
    prepare unless @prepared
    @conn.exec_prepared(@stmt_name, (@query.arel.bind_values + @query.bind_values).map { |c, value| c.cast_type.type_cast_for_database(value) })
  end

  def prepare
    @conn.prepare(@stmt_name, "DECLARE #{@cursor_name} CURSOR FOR #{@query.arel.to_sql}")
    @prepared = true
  end

  def fetch
    @observer.fetching(@fetch_size)
    result = @conn.exec("FETCH #{@fetch_size} IN #{@cursor_name}")
    result.collect
  end

  def close
    @conn.exec("CLOSE #{@cursor_name}")
  rescue => e
    @observer.close_failed(e)
  end
end