I searched all the documentation over Yii but not got the answer of it.So I came here finally. I have the following schema

Table Schools
+------------------+--------------+------+-----+---------------------+----------------+
| Field            | Type         | Null | Key | Default             | Extra          |
+------------------+--------------+------+-----+---------------------+----------------+
| id               | int(10)      | NO   | PRI | NULL                | auto_increment |
| school_name      | varchar(100) | NO   |     |                     |                |
+------------------+--------------+------+-----+---------------------+----------------+

Table Students

+------------------+--------------+------+-----+---------------------+----------------+
| Field            | Type         | Null | Key | Default             | Extra          |
+------------------+--------------+------+-----+---------------------+----------------+
| id               | int(10)      | NO   | PRI | NULL                | auto_increment |
| school_id        | int(10)      | NO   | FK  |                     |                |
| student_name     | varchar(100) | NO   |     |                     |                |
| roll_no          | varchar(80)  | NO   |     |                     |                |
| class            | varchar(20)  | NO   |     |    |                |                |
| subjects         | varchar(100) | NO   |     |                     |                |
+------------------+--------------+------+-----+---------------------+----------------+

I have made models and CRUD for the both models.In models my relation is like this

In Students.php the relation is like

  public function relations()
  {
    // NOTE: you may need to adjust the relation name and the related
    // class name for the relations automatically generated below.
    return array(
      'School' => array(self::BELONGS_TO,'Schools','school_id'),
    );
  }

In Schools.php the relation is like

public function relations()
  {
    // NOTE: you may need to adjust the relation name and the related
    // class name for the relations automatically generated below.
    return array(
      'student' => array(self::HAS_MANY, 'Students', 'school_id'),
    );
  }

Now I made the two models rendered in a single page so that I can enter all the respective fields in a single form.

 In the _form.php file of Students I have made some change in student_name like this
         <div class="row">
        <?php echo $form->labelEx($model,'student_name'); ?>
        <?php echo $form->dropdownList($model,'student_name', CHtml::listData(Students::model()->findAll(), 'id', 'student_name'), array('empty'=>array('Select'=>'--Select One---'))); ?>
        <?php echo $form->error($model,'student_name'); ?>

Now for this piece of code I got all the student name from the Student model. So my problem is when I am getting the student name from the dropdown list and going to select a student it will also fetch all the respective values of the student to be rendered in the _form.php without click on save button.So that user don't have to put it again manually. I think ajax and json encode will work here but don't know how to make them work here.

[Update]

Here is StudentsController code

 public function actionDisCoor() {
    $model = School::model()->findByPk($_POST['Students']['student_id']);
    $data=CHtml::listData($data,'id','name');
    foreach($data as $value=>$name)
    {
      echo CHtml::tag('option',array('value'=>$value),CHtml::encode($name),true);
    }
  }

Here is _form.php code for Students

  <div class="row">
    <?php echo $form->labelEx($model,'student_name'); ?>
    <?php  $List = CHtml::listData(Students::model()->findAll(), 'id', 'student_name');
?>
    <?php echo $form->dropdownList($model,'student_name',$List,
                                array('onChange'=>CHtml::ajax(array(
                                'url' => CController::createUrl('DisCoor'),
                                'type' => 'POST',                     
                               'update'=>'#school_id',
                                )),'style'=>'width:180px;'
                                    )
                                )?>
    <?php echo $form->error($model,'student_name'); ?>
  </div>

After all that when I saw in firebug I got the error.Here is the screen shot