java - eclipselink 不获取列表

Question

有两个实体类:

@Entity
public class Place implements Serializable {
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY) private Long id;
    @OneToMany(mappedBy = "place")
    private List<Event> events = new ArrayList<Event>();
    private String name;
//getters setters
}
@Entity
public class Event implements Serializable {
    @Id @GeneratedValue(strategy=GenerationType.IDENTITY) private Long id;
    @OneToMany(mappedBy = "event" , cascade={CascadeType.PERSIST})
    private List<Participant> participants = new ArrayList<Participant>();
    @ManyToOne(optional=false,fetch=FetchType.EAGER) private Place place;
    private String name;
// getters setters
}

生成的表格:

mysql> desc EVENT;
+----------+--------------+------+-----+---------+----------------+
| Field    | Type         | Null | Key | Default | Extra          |
+----------+--------------+------+-----+---------+----------------+
| ID       | bigint(20)   | NO   | PRI | NULL    | auto_increment |
| NAME     | varchar(255) | YES  |     | NULL    |                |
| PLACE_ID | bigint(20)   | YES  | MUL | NULL    |                |
+----------+--------------+------+-----+---------+----------------+
mysql> desc PLACE;
+----------+--------------+------+-----+---------+----------------+
| Field    | Type         | Null | Key | Default | Extra          |
+----------+--------------+------+-----+---------+----------------+
| ID       | bigint(20)   | NO   | PRI | NULL    | auto_increment |
| LOCATION | varchar(255) | YES  |     | NULL    |                |
| NAME     | varchar(255) | YES  |     | NULL    |                |
+----------+--------------+------+-----+---------+----------------+

现在，我将编写一些 Java 代码，使 Place 和 Event 表看起来像这样:

+----+--------+
| ID | NAME   |
+----+--------+
|  1 | Prague |
+----+--------+
+----+-------------+----------+
| ID | NAME        | PLACE_ID |
+----+-------------+----------+
|  1 | Programming |        1 |
+----+-------------+----------+

现在，我想选择一个地方并查看它的所有事件:

// there's an ejb talking with persistence layer 
Place p = ejb.getPlaceWithId(1);

而且我认为 p.events 中除了空集合之外还有其他东西，但根据我的观察，没有。为什么？

Answer 1

Now, I would like to pick a place and view all its events (...) and I would think that there's something more than empty collection in p.events, but there is not - based on my observations. Why?

我的观察是您显示的代码中的事件是LAZY:

@OneToMany(mappedBy = "place")
private List<Event> events = new ArrayList<Event>();

那么，当准确加载给定的 Place 时，您是如何得出它们为空的结论的？您是否尝试对集合调用 size() 以强制加载(或迭代集合项)？

不管上面问题的答案如何，我的建议是

• 激活 SQL 日志记录(并在必要时在 SQL 客户端中运行查询)以查看发生了什么。

• 如果您想急切地加载事件，也许实际上可以使用fetch join:

  SELECT p FROM Place p join fetch p.events WHERE p.id = :id