It's common to only list records if there are any – something like this:

<% if @items.any? %>
  <ul>
    <% @items.each do |item| %>
      <li><%= item.title %></li>
    <% end %>
  </ul>
<% else %>
  <p>No items!</p>
<% end %>

As you may be currently shouting at your screen, this specific implementation is not optimal. The any? will trigger one query, and @items.each will trigger another.

We can verify this in a Rails console:

>> items = Item.where("false"); items.any?; items.each(&:id)
  Item Exists (1.2ms)  SELECT  1 AS one FROM "items" WHERE (false) LIMIT 1
  Item Load (12.3ms)  SELECT "items".* FROM "items" WHERE (false)

Other than the performance implications of running two queries where one would do, there's also a (usually low) risk of timing issues where the any? is true, then the records are destroyed, and the second query comes back empty, rendering an empty list.

The docs currently recommend using length, e.g. like this:

>> items = Item.where("false"); items.length.nonzero?; items.each(&:id)
  Item Load (12.3ms)  SELECT "items".* FROM "items" WHERE (false)

This does get us a single query, but at what aesthetic cost? @items.length.nonzero? or @items.length > 0 lacks the elegance of @items.any?. Surely the framework that gave us #forty_two can do better!

So Calle and I looked around and came up with something that gets us both:

<% if @items.load.any? %>
  <ul>
    <% @items.each do |item| %>
      <li><%= item.title %></li>
    <% end %>
  </ul>
<% else %>
  <p>No items!</p>
<% end %>

By just sneaking a load in there, we cause the @items collection to be loaded from the database. And any? is implemented (via empty?) not to fire off an extra query if the collection has already been loaded.

We can confirm in the console that it generates a single query:

>> items = Item.where("false"); items.load.any?; items.each(&:id)
  Item Load (12.3ms)  SELECT "items".* FROM "items" WHERE (false)

Alternatively, one could load the records in the controller, of course:

@items = Item.some_scope.load

A potential downside to doing it in the controller is that it still looks in the view like it will trigger an extra query, if you're used to this Rails gotcha…

Anyway, that's it!

UPDATE: After writing this, I found an excellent post by Nate Berkopec that covers a bunch of options (including this one!) in depth, with tables and everything. Read it too!