Join from two tables to composite key in third table with JPA

Question

I want to use Hibernate with JPA to do a join between three tables, like this:

Tables: [primary key in square brackets]

Worker: ([worker_id], tool_id, site_id)
Toolbelt: ([toolbelt_id])
Site: ([site_id])
SiteToolbelt: ([site_id], toolbelt_id)
Tool: ([toolbelt_id, tool_id], tool_name)

To be clear, this means that tools on different toolbelts having the same tool_id can have different names.

This is the SQL query I want:

SELECT * FROM Worker w
JOIN SiteToolbelt st ON w.site_id = st.site_id
JOIN Tool t ON t.toolbelt_id = st.toolbelt_id AND t.tool_id = w.tool_id

How do I do such a thing with JPA?

Answer 1

From my prespective that tool_id is out of place, because you cant get tool from only its tool_id, its better to call that tool_type or something like that. In this case just use embeddable class as composite id

you should have class like this

@Embeddable
public class ToolPK implements Serializable {
    @ManyToOne
    @JoinColumn(name = "TOOLBELT_ID")
    private Toolbelt toolbelt;
    private Long toolType;
//(getter setter here)
}

Then use it in the Tool class as primary key

@EmbeddedId
private ToolPK toolPK;
//this is the real tool_id 
//not like tool_type which cant get you the object Tool without the toolbelt

Then you only need to put tool_type in the user, not tool_id(because tool_id is composite).