Zend Framework - Database Table Singleton

I have found myself doing this in my code to 'cache' the work done when instantiating my Zend_Db_Table models:

if (Zend_Re开发者_如何学编程gistry::isRegistered('x_table')) {
    $x_table = Zend_Registry::get('x_table');
} else {
    $x_table = new Default_Model_DbTable_X;
    Zend_Registry::set('x_table', $x_table);
}

It bothered me that this method isn't very DRY and it dawned on me today that a singleton pattern would probably be a better way to do this. Problem is, I've never written a singleton class. When I did some web searches, I found some offhand comments about Zend_Db_Table singletons, but no real examples.

I already have meta-data caching configured.

1. How do I make my Zend_Db_Table models singletons?
2. Are there pitfalls or downsides?

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Edit: My reason for thinking a singleton was the answer is that I thought I could simply have the following calls in my code $x_table = new Default_Model_DbTable_X; and the single instance would be returned if it existed. If this is possible, I would prefer that solution.

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Why not just manage the DbTables in some sensible place? If there's no sensible place, create a DbTableManager class. Something like:

<?PHP
class DbTableMgr {

  protected $_tables;

  public function getTable($classname){
    if ( empty($this->_tables[$name]) ){
      //assuming some things about class names for the sake of brevity elsewhere...
      $classname = 'Default_Model_DbTable_',ucfirst(strtolower($classname));

      $this->_tables[$name] = new $classname; 
    }
    return $this->_tables[$name];  
  }
}

Initialize the manager in your bootstrap and stick it in the registry.

Then:

<?PHP
//in a galaxy far, far, away
$dbtFoo = Zend_Registry::get('dbtMgr')->getTable('Foo');

So, you lazy-load the dbTable objects, and enforce a singleton-like behavior.

You could make the above static in various ways, if you wanted to.