Yii2: auto fill fields based on another field from

I have a MySQL table and model patient_entry which contains fields patient_name, city and state. I also have another table/model health_card which also contains patient_name, city and state.

Suppose the patient_entry table is already filled with patient_name, city and state.

When I am entering data in health_card form, when I select the patient_name via drop-down field related to patient_entry table, I want the related city and state fields to be auto-filled.

My _form.php for health_card looks like this:

    <head>
<script>

        $this->registerJs("$('#healthcard-patient_name').on('change',function(){
    $.ajax({
        url: '".yii\helpers\Url::toRoute("HealthCard/patient")."',
        dataType: 'json',
        method: 'GET',
        data: {id: $(this).val()},
        success: function (data, textStatus, jqXHR) {
            $('#healthcard-city').val(data.city);
            $('#healthcard-pincode').val(data.pin);
        },
        beforeSend: function (xhr) {
            alert('loading!');
        },
        error: function (jqXHR, textStatus, errorThrown) {
            console.log('An error occured!');
            alert('Error in ajax request');
        }
    });
});"); 

      </script>
</head>

And in the controller I have added, as per the suggestion, this:

public function actionPatient($id){
    // you may need to check whether the entered ID is valid or not
    $model= \app\models\PatientEntry::findOne(['id'=>$id]);
    return \yii\helpers\Json::encode([
        'city'=>$model->disrict_city,
        'pin'=>$model->pin_code
    ]);
}

All you need is calling an AJAX request to get needed fields. Just act like below:

(I do not know your model name)take a look at your form and see what is the id of your patient_name field. It is usually modelname-fieldname. I assume that your model name is Patient. So, the id of patient_name would be patient-patient_name.
Add an ajax request (in your view).

The code for calling AJAX could look just like below:

$this->registerJs("$('#patient-patient_name').on('change',function(){
    $.ajax({
        url: '".yii\helpers\Url::toRoute("controllerName/patient")."',
        dataType: 'json',
        method: 'GET',
        data: {id: $(this).val()},
        success: function (data, textStatus, jqXHR) {
            $('#patient-city').val(data.city);
            $('#patient-state').val(data.state);
        },
        beforeSend: function (xhr) {
            alert('loading!');
        },
        error: function (jqXHR, textStatus, errorThrown) {
            console.log('An error occured!');
            alert('Error in ajax request');
        }
    });
});");

Notes:

Change the ControllerName in above code with your own.
I assumed that the id of city and state fields has the following id(s): patient-city and state-city relatively.
patient is an action in your controller
You may need to remove alerts|logs and do some customization on above code
I didn't consider any conditions for code cleaning. Please make sure that user data is correct.
1. Finally, add action code into your controller.

Action code:

public function actionPatient($id){
    // you may need to check whether the entered ID is valid or not
    $model=  \app\models\Patient::findOne(['id'=>$id]);
    return \yii\helpers\Json::encode([
        'city'=>$model->city,
        'state'=>$model->state
    ]);
}