How to query many to many relationship? - jpa

Question

So I got many to many relationship in two different entities. One has a list of the other and the other has a list of the first one. My question is how to make query that takes one field of the entity + the collection. For example Professors and Projects are related many to many, how to get the name of all professors + the projects they are involved in? Please if you have ideas share deeply and broadly. Thanks

Answer 1

You can use JP QL Projections, select on the Professor's name plus the properties of the Project entity associated with the professor. Using your example:
#Entity
public class Professor {
private Long id;
private String name;
private Collection<Projects> projects;
...
}
#Entity
public class Project {
private Long id;
private String status;
private Collection<Professor> professors;
...
}
Sample data:
Your Professor table has 2 rows:
1 Mark
2 John
Your Project table has 4 rows:
1 A DELAYED
2 B COMPLETED
3 C ON-TRACK
4 D COMPLETED
The JOIN table (the first column is the Professor ID, 2nd column is the Project ID):
1 1
1 2
2 3
2 4
Your JPA Query will be like:
SELECT prof.name, proj.name, proj.status FROM Professor prof JOIN prof.projects proj
Then the results will be:
Mark A DELAYED
Mark B COMPLETED
John C ON-TRACK
John D COMPLETED
However, if you're trying use projection on the Professor name plus its project collections, that will not be possible in JP QL. JP QL only allows single-valued path expression in the SELECT clause.
Example: This is NOT A VALID JP QL Query, since prof.projects is a collection-valued path expression.
SELECT prof.name, prof.projects FROM Professor prof