Question

How can I join two tables by using java play framework and jpa, I really have a hardtime converting my MySQL query to jpa query.

Here is the MySQL query that I used in my old Java code:

SELECT * FROM tbl_majors
INNER JOIN tbl_lookup_user_major
ON tbl_majors.id=tbl_lookup_user_major.majorId
WHERE tbl_lookup_user_major.userId=12

//Table 1:

@Entity
@Table(name="tbl_majors")
public class Major extends Model {
    public Major(){

    }
    @Column(name="major_name")
    private String name;
    @Column(name="major_desc")
    private String description;
}

//Table 2

@Entity
@Table(name="tbl_lookup_user_major")
public class LookupUserMajor extends Model {
    public LookupUserMajor(){

    }
    private int majorId;
    private int userId;
}

Answer 1

Dont know if I get the exact point here, but in the tutorial blog “YABE”, this kind of join table is used and created automatically by Play :

http://www.playframework.org/documentation/1.2.4/guide6#tagging

The many-to-many relation is described in the Model (between “Post” and “Tag” here for the blog sample) :

@ManyToMany(cascade=CascadeType.PERSIST)
public Set<Tag> tags;

public Post(User author, String title, String content) {
    ...
    this.tags = new TreeSet<Tag>();
    ...
    this.title = title;
    this.content = content;
    ...
}

The YAML for the Posts data is :

Post(jeffPost):
    title:          The MVC application
    postedAt:       2009-06-06
    author:         jeff
    tags:           
                    - play
                    - architecture
                    - mvc

After running the app, I check the database and the table “post_tag” is automatically created and all the links between the two tables are done (post_ids and tags_ids are filled).

Retrieving data seems as easy as :

"select distinct p from Post p join p.tags as t"

Can someone confirm that ? Because new to Java and JPA and Play ^^

If this is correct, it looks easier than managing the join table “manually”.