// declare own provider class
// assume there are tables and InfoBase classes like those in constructor's $knownTables
class MyDataProvider extends DataProviderBase
{

  // declare singleton instance
  private static $instance;
  static function instance()
  {
    if (!self::$instance)
      self::$instance = new MyDataProvider ();
    return self::$instance;
  }

  // protected constructor
  protected function __construct()
  {
    $knownTables = array(
      'MyPersonInfo' => 'persons',
      'MyCompanyInfo' => 'companies',
      'MyRelationInfo' => 'person2company'
    );
    parent::__construct($knownTables);
  }

  // implement some simple retrieving functions

  function GetPerson ($personId)
  {
    return $this->GetItem('MyPersonInfo', $personId);
  }

  function GetCompany ($companyId)
  {
    return $this->GetItem('MyCompanyInfo', $companyId);
  }

  // implement some more complex functions

  function HireEmployee (MyCompanyInfo $company, MyPersonInfo $employee)
  {
    // check if already employed
    $key = new SearchKey('id_company', $company->ID());
    $key->AddAndKey(new SearchKey('id_employee', $employee->ID()));
    // look for such relation
    if (!$this->ListItems('MyRelationInfo', $key))
    {
      // not employed yet, create relation
      $rel = new MyRelationInfo ($employee->ID(), $company->ID());
      $this->UpdateItem($rel);
      return true; // we have hired him
   }
  }

  function FireEmployee (MyCompanyInfo $company, MyPersonInfo $employee)
  {
    // check if employed
    $key = new SearchKey('id_company', $company->ID());
    $key->AddAndKey(new SearchKey('id_employee', $employee->ID()));
    // look for such relation
    if ($list = $this->ListItems('MyRelationInfo', $key))
    {
      // is employed, delete relation
      $this->DeleteItem($list[0]);
      return true; //we have fired him
    }
  }

  function ListEmployees (MyCompanyInfo $company)
  {
    // PersonInfo->id is related to MyRelationInfo->id_person
    $rel = new SearchRelation ('id', 'MyRelationInfo', 'id_person');
    // query where MyRelationInfo->id_company points to our $company
    $key = new RelatedKey ($rel, 'id_company', $company->ID());
    // get all such MyPersonInfo instances
    return $this->ListItems ('MyPersonInfo', $key);
  }

  // implement some transaction Update functions
  // assume MyPersonInfo has a function Company() returning CompanyInfo or null

  function UpdatePerson (MyPersonInfo $person)
  {
    $trans = $this->BeginTransaction();
    try
    {
      if ($person->isDirty)
        // update $person itself
        $this->UpdateInfo ($person);
      if ($company = $person->Company())
      {
        // first update $company as it could be a new one
        $this->UpdateCompany ($company);
        // now ensure $person is employed there
        $this->HireEmployee ($company, $person);
      }
      // OK, commit
      $this->CommitTransaction ($trans);
    }
    catch (Exception $ex)
    {
      // something went wrong, rollback and throw again
      $this->RollbackTransaction ($trans);
      throw $ex;
    }
  }

  function UpdateCompany (MyCompanyInfo $company)
  {
    if ($company->isDirty) // else ignore
    {
      $trans = $this->BeginTransaction();
      try
      {
        // update $company
        $this->UpdateItem($company);
        // do what else necessary
        // ...

        // and commit
        $this->CommitTransaction ($trans);
      }
     catch (Exception $ex)
      {
        // something went wrong, rollback and throw again
        $this->RollbackTransaction ($trans);
        throw $ex;
      }
    }
  }