If you define multiple `@ OneToMany``` in JPA and try to fetch both with
`join fetch```, the following run-time exception may occur.
Caused by: org.hibernate.loader.MultipleBagFetchException: cannot simultaneously fetch multiple bags: [jpa.sample2.Tweet.reTweets, jpa.sample2.Tweet.favorites]
at org.hibernate.loader.BasicLoader.postInstantiate(BasicLoader.java:76) ~[hibernate-core-5.4.20.Final.jar:5.4.20.Final]
at org.hibernate.loader.hql.QueryLoader.<init>(QueryLoader.java:110) ~[hibernate-core-5.4.20.Final.jar:5.4.20.Final]
As a schema for explanation, assume a table of multiple ReTweets and multiple Favorites in one tweet. The entity of java is as follows.
@NoArgsConstructor
@AllArgsConstructor
@Entity
@Data
public class Tweet {
@Id
Long id;
String value;
@OneToMany
@JoinColumn(name = "tweetId")
List<ReTweet> reTweets;
@OneToMany
@JoinColumn(name = "tweetId")
List<Favorite> favorites;
}
@NoArgsConstructor
@AllArgsConstructor
@Entity
@Data
public class ReTweet {
@Id
Long id;
Long tweetId;
String value;
}
@NoArgsConstructor
@AllArgsConstructor
@Entity
@Data
public class Favorite {
@Id
Long id;
Long tweetId;
String value;
}
JPQL uses `` `join fetch``` twice as shown below.
@Repository
public interface TweetRepository extends JpaRepository<Tweet, Long> {
@Query("select distinct t from Tweet t "
+ " left join fetch t.reTweets "
+ " left join fetch t.favorites"
+ " order by t.id")
List<Tweet> list2();
}
This will result in the above run-time exception.
In this case, change `List``` to
`Set```.
@OneToMany
@JoinColumn(name = "tweetId")
Set<ReTweet> reTweets;
@OneToMany
@JoinColumn(name = "tweetId")
Set<Favorite> favorites;
Or it may be ``` Map``.
@OneToMany
@JoinColumn(name = "tweetId")
Map<Long, ReTweet> reTweets;
@OneToMany
@JoinColumn(name = "tweetId")
Map<Long, Favorite> favorites;
Alternatively, it is possible to give up a single JPQL and divide it into multiple parts.
@Query("select distinct t from Tweet t "
+ " left join fetch t.reTweets "
+ " order by t.id")
List<Tweet> list3();
@Query("select distinct t from Tweet t "
+ " left join fetch t.favorites"
+ " order by t.id")
List<Tweet> list4();