title: counter_cache has_many_through sql optimisation, reduce number of sql queries
body: <p>How I can optimise my SQL queries, to ignore situations like this:</p>
  <blockquote>
  <p>Meeting.find(5).users.size =&gt; SELECT COUNT(*) FROM ... WHERE ...</p>
  <p>User.find(123).meetings.size =&gt; SELECT COUNT(*) FROm ... WHERE ...</p>
  </blockquote>
  <p>I have no idea how to use counter_cache here.</p>
  <p>Here is my model relation:</p>
  <pre><code>class Meeting &lt; ActiveRecord::Base
    has_many :meeting_users
    has_many :users, :through =&gt; meeting_users
  end
  
  class User &lt; ActiveRecord::Base
    has_many :meeting_users
    has_many :meetings, :through =&gt; meeting_users
  end
  
  class Meeting_user &lt; ActiveRecord::Base
    belongs_to :meeting
    belongs_to :user
  end
  </code></pre>
  <p>What are the most optimal solutions ?</p>
  <p>And how implement counter_cache here ?</p>