title: Deprecation warning when using has_many :through :uniq in Rails 4
body: <p>Rails 4 has introduced a deprecation warning when using :uniq => true with has_many :through. For example: </p>
  
  <pre><code>has_many :donors, :through =&gt; :donations, :uniq =&gt; true
  </code></pre>
  
  <p>Yields the following warning: </p>
  
  <pre><code>DEPRECATION WARNING: The following options in your Goal.has_many :donors declaration are deprecated: :uniq. Please use a scope block instead. For example, the following:
  
      has_many :spam_comments, conditions: { spam: true }, class_name: 'Comment'
  
  should be rewritten as the following:
  
      has_many :spam_comments, -&gt; { where spam: true }, class_name: 'Comment'
  </code></pre>
  
  <p>What is the correct way to rewrite the above has_many declaration? </p>