title: Active Record has_many :through remove one associated record
body: <p>This may be a very basic oversight on my part, but I can't seem to recall a simple method for removing an association between two objects joined via <code>has_many :through</code>. IE:</p>
  
  <pre><code>class Photo
    has_many :tags, :through =&gt; :taggings
    has_many :taggings, :dependent =&gt; :destroy
  end
  
  class Tags
    has_many :photos, :through =&gt; :taggings
    has_many :taggings, :dependent =&gt; :destroy
  end
  
  class Taggings
    belongs_to :photo
    belongs_to :tag
  end
  </code></pre>
  
  <p>If you have two objects, <code>tag</code> and <code>photo</code>, you can associate them just by doing this:</p>
  
  <pre><code>photo.tags &lt;&lt; tag
  </code></pre>
  
  <p>So, is there an equally simple opposite to this? ie:</p>
  
  <pre><code>photo.tags.remove tag
  </code></pre>