title: Composite relations (relations built out of multiple separate queries)
body: <p>Let's take two simple queries:</p>
  
  <pre><code>@c = @project.companies.where("companies.public = ?", true)
  @i = @item.companies.where("companies.created_at &gt; ?", Date.today - 3.days).limit(3)
  </code></pre>
  
  <p>Say I want to combine these into a single relation of companies, which I then want to paginate and search on. Let's try:</p>
  
  <pre><code>@companies = (@c + @i).paginate...
  # or #
  @companies = (@c &lt;&lt; @i).paginate...
  </code></pre>
  
  <p>Both of these fail because <code>@companies</code> is an array, not a relation. How can two or more relations be combined to form a third relation?</p>