title: How to limit number of accociations in Rails
body: <p>Straight to subject: I have Game, Board and Player models in my app. </p>
  
  <pre><code>Game -&gt; Board &lt;- Player
  </code></pre>
  
  <p>Game to player is many to many relation through Boards. I'd like to limit that game can have only 2 Boards (therefore only two players).</p>
  
  <pre><code>game = Game.create
  
  game.players.push Player.create
  game.players.push Player.create
  game.players.push Player.create #this line should throw some exception
  </code></pre>
  
  <p>I don't see anything out of the box I could use. One idea is to use validations, but is this the only way?</p>