Activerecord cant update a row - yii2

Question

I want update just one column of table I'm getting this error when updating a table rows:
Call to a member function load() on null
This is my action:
public function actionAddNote(){
$id = \Yii::$app->request->post('id');
$model = MainRequest::findOne($id);
if ($model->load(\Yii::$app->request->post()) && $model->validate())
{
if($model->update()){
echo "1";
}else {
print_r($model->getErrors());
}
}
return $this->renderAjax('add-extra-note',['model' => $model]);
}
My model:
class MainRequest extends ActiveRecord {
public static function tableName()
{
return "main_request";
}
public function behaviors()
{
return [
DevNotificationBehavior::className(),
];
}
public function rules() {
return [
[
['who_req',
'req_description',
'req_date',
'extra_note'
], 'safe']
];
}
The form will render properly and I can see my text but when I submit this the error occur:
<div>
<?php $form = ActiveForm::begin(); ?>
<?= $form->field($model, 'extra_note')->textInput(); ?>
<div class="form-group">
<?= Html::submitButton('save', ['class' => 'btn green']) ?>
</div>
<?php ActiveForm::end(); ?>
</div>
Can anybody tell what is problem ? Thank you.

Answer 1

You should load the model and the use the model loaded for accessing attribute
and you should manage ythe initial situation where you d0n't have a model to update but need a model for invoke the update render form eg:
public function actionAddNote(){
$myModel = \Yii::$app->request->post();
$model = MainRequest::findOne($myModel->id);
if (isset($model)){
if ($model->load(\Yii::$app->request->post()) && $model->validate())
{
if($model->update()){
echo "1";
}else {
print_r($model->getErrors());
}
}
} else {
$model = new MainRequest();
}
return $this->renderAjax('add-extra-note',['model' => $model]);
}

Answer 2

Use Simple save function or updateAttributes of Yii2:
public function actionAddNote(){
$id = \Yii::$app->request->post('id');
$model = MainRequest::findOne($id);
if ($model->load(\Yii::$app->request->post()) && $model->validate())
{
if($model->**save**()){
echo "1";
}else {
print_r($model->getErrors());
}
}
return $this->renderAjax('add-extra-note',['model' => $model]);
}