title: Eloquent morphOne relationship doesn't limit to one relation
body: <p>I am having a issue with an Eloquent <code>morphOne</code> relationship where it is creating new entries rather than updating the one that already exists.</p>
  
  <p>Basically I have a number of models (for example, let's say <code>Person</code> and <code>Building</code>) that both need a location, so I have created a <code>Location</code> model:</p>
  
  <pre><code>class Location extends Eloquent {
  
      public function locationable()
      {
          return $this-&gt;morphTo();
      }
  
  }
  </code></pre>
  
  <p>Then in my other models I have this:</p>
  
  <pre><code>class Person extends Eloquent {
  
      // ...
  
      /**
       * Get the person's location
       * 
       * @return Location
       */
      public function location()
      {
          return $this-&gt;morphOne('Location', 'locationable');
      }
  
      // ...
  </code></pre>
  
  
  
  <pre><code>class Building extends Eloquent {
  
      // ...
  
      /**
       * Get the building's location
       * 
       * @return Location
       */
      public function location()
      {
          return $this-&gt;morphOne('Location', 'locationable');
      }
  
      // ...
  </code></pre>
  
  <p>When I run the following test code, it creates the location entry fine, but if I repeat it, it creates more entries.</p>
  
  <pre><code>$person = Person::first();
  
  $loc = new Location;
  
  $loc-&gt;lat = "123";
  $loc-&gt;lng = "321";
  
  $person-&gt;location()-&gt;save($loc);
  </code></pre>
  
  <p>Am I doing something wrong here? I would have expected <code>morphOne</code> to constrain this to one entry per type, so the last entry in the table below should not exist:</p>
  
  <pre><code>+---------------------+--------------------------+
  |  locationable_id    |   locationable_type      |
  +---------------------+--------------------------+
  |  2                  |  Building                |
  |  3                  |  Building                |
  |  2                  |  Person                  |
  |  2                  |  Building                |
  +---------------------+--------------------------+
  </code></pre>