title: on a has_many :through association, replacing conditions with lambda
body: <p>I have this association:</p>
  
  <pre><code>has_many :foo_participators, through: :foos, source: :user, conditions: "foos.state = 'completed'"
  </code></pre>
  
  <p>Rails tells me:</p>
  
  <blockquote>
    <p>DEPRECATION WARNING: The following options in your Bar.has_many
    :foo_participators declaration are deprecated: :conditions. Please use a scope
    block instead. For example, the following:</p>
  
  <pre><code>has_many :spam_comments, conditions: { spam: true }, class_name: 'Comment'
  </code></pre>
    
    <p>should be rewritten as the following:</p>
  
  <pre><code>has_many :spam_comments, -&gt; { where spam: true }, class_name: 'Comment'
  </code></pre>
  </blockquote>
  
  <p>Is this possible with my association?</p>