私信的设计

1. 发件人和消息的关系 (一对多)
2. 消息和收件人的关系 (多对多)

pre_messages

+------------+------------------+------+-----+---------+----------------+
| Field      | Type             | Null | Key | Default | Extra          |
+------------+------------------+------+-----+---------+----------------+
| id         | int(11) unsigned | NO   | PRI | NULL    | auto_increment |
| sender_id  | int(11) unsigned | NO   |     | 0       |                |
| message    | varchar(255)     | NO   |     |         |                |
| type       | varchar(50)      | NO   |     |         |                |
| expires_at | datetime         | YES  |     | NULL    |                |
| created_at | datetime         | NO   |     | NULL    |                |
| updated_at | datetime         | NO   |     | NULL    |                |
| deleted_at | datetime         | YES  |     | NULL    |                |
+------------+------------------+------+-----+---------+----------------+

pre_message_recipients

+--------------+---------------------+------+-----+---------+----------------+
| Field        | Type                | Null | Key | Default | Extra          |
+--------------+---------------------+------+-----+---------+----------------+
| id           | int(11) unsigned    | NO   | PRI | NULL    | auto_increment |
| recipient_id | int(11) unsigned    | NO   |     | 0       |                |
| message_id   | int(11) unsigned    | NO   |     | 0       |                |
| status       | tinyint(1) unsigned | NO   |     | 0       |                |
| created_at   | datetime            | NO   |     | NULL    |                |
| updated_at   | datetime            | NO   |     | NULL    |                |
| deleted_at   | datetime            | YES  |     | NULL    |                |
+--------------+---------------------+------+-----+---------+----------------+