yii2-save-relations-behavior/tests/models/UserProfile.php

<?php

namespace tests\models;

class UserProfile extends \yii\db\ActiveRecord
{
    /**
     * @inheritdoc
     */
    public static function tableName()
    {
        return 'user_profile';
    }

    /**
     * @inheritdoc
     */
    public function rules()
    {
        return [
            [['user_id'], 'integer'],
            ['bio', 'required'],
            [['user_id'], 'unique'],
            [['user_id'], 'exist', 'skipOnError' => true, 'targetClass' => User::className(), 'targetAttribute' => ['user_id' => 'id']]
        ];
    }

}