title: Get a single record from a record set
body: <p>Simple request:</p>
  
  <pre><code>    a1 = where(MyModel, [x], is_nil(x.some_foreign_id)) |&gt; Repo.one()
  </code></pre>
  
  <p>Exception:</p>
  
  <pre><code>** (exit) an exception was raised:                                
      ** (Ecto.MultipleResultsError) expected at most one result but got 6 in query:
  </code></pre>
  
  <p>Yes, there're a few records and that's ok, but my intention is to take the 1st one. Why exception and how to fix it?</p>