CExistValidator

protected function getModel($className)
{
    return CActiveRecord::model($className);
}

protected function validateAttribute($object,$attribute)
{
    $value=$object->$attribute;
    if($this->allowEmpty && $this->isEmpty($value))
        return;

    if(is_array($value))
    {
        // https://github.com/yiisoft/yii/issues/1955
        $this->addError($object,$attribute,Yii::t('yii','{attribute} is invalid.'));
        return;
    }

    $className=$this->className===null?get_class($object):Yii::import($this->className);
    $attributeName=$this->attributeName===null?$attribute:$this->attributeName;
    $finder=$this->getModel($className);
    $table=$finder->getTableSchema();
    if(($column=$table->getColumn($attributeName))===null)
        throw new CException(Yii::t('yii','Table "{table}" does not have a column named "{column}".',
            array('{column}'=>$attributeName,'{table}'=>$table->name)));

    $columnName=$column->rawName;
    $criteria=new CDbCriteria();
    if($this->criteria!==array())
        $criteria->mergeWith($this->criteria);
    $tableAlias = empty($criteria->alias) ? $finder->getTableAlias(true) : $criteria->alias;
    $valueParamName = CDbCriteria::PARAM_PREFIX.CDbCriteria::$paramCount++;
    $criteria->addCondition($this->caseSensitive ? "{$tableAlias}.{$columnName}={$valueParamName}" : "LOWER({$tableAlias}.{$columnName})=LOWER({$valueParamName})");
    $criteria->params[$valueParamName] = $value;

    if(!$finder->exists($criteria))
    {
        $message=$this->message!==null?$this->message:Yii::t('yii','{attribute} "{value}" is invalid.');
        $this->addError($object,$attribute,$message,array('{value}'=>$value));
    }
}