ActiveRecord保存之后取不到id的问题

common\models\User#1
(
    [yii\db\BaseActiveRecord:_attributes] => [
        'username' => 'test5'
        'email' => 'test01@qq.com'
        'role' => 10
        'password_hash' => '$2y$13$CfGBbRQJ5kgiEM01sXg0K.0dbErf80spIuHnhAsrhdsUOF61ryKZG'
        'auth_key' => 'tXqybSQ9LLtzvw-XyH9dGvkOp6yJnfWZ'
        'status' => 10
        'created_at' => 1467054625
        'updated_at' => 1467054625
        'id' => null
    ]
    [yii\db\BaseActiveRecord:_oldAttributes] => [
        'username' => 'test5'
        'email' => 'test01@qq.com'
        'role' => 10
        'password_hash' => '$2y$13$CfGBbRQJ5kgiEM01sXg0K.0dbErf80spIuHnhAsrhdsUOF61ryKZG'
        'auth_key' => 'tXqybSQ9LLtzvw-XyH9dGvkOp6yJnfWZ'
        'status' => 10
        'created_at' => 1467054625
        'updated_at' => 1467054625
        'id' => null
    ]
}

为什么保存之后id值为空了？