title: Rails3: how to set default conditions to has_many
body: <p>I have boxes and balls. Balls are in boxes. Ball can be either red and green.</p>
  
  <pre><code>class Box &lt; ActiveRecord::Base
    has_many :balls
  end
  
  class Ball &lt; ActiveRecord::Base
    belongs_to :box
    scope :green, where(:color =&gt; "green")
  end
  </code></pre>
  
  <p>I want to set has_many only with green balls. I know that finder_sql method exists, but i don't know how to set via scopes.</p>
  
  <p>I want that following examples to be equivalent:</p>
  
  <pre><code>@orders = @box.balls
  @orders = @box.balls.green
  </code></pre>