Hibernate - One to Many associations

Question

i have two tables, ComputerNode and Connection. ComputerNode has a primary key nodeid but Connection do not have one. I can't modify the table schema. How should i create the java POJO if they were to have one-to-many relationship? basically my objective is to do a inner join like this:

select * from `ComputerNode` cn inner join `Connection` c on cn.nodeid = c.nodeid

Here are the SQL table schema. ComputerNode table:

int nodeid <primary key>;
varchar nodename;

Connection table:

int nodeid <not primary key>;
varchar connstatus;

The relationship between the tables is one-to-many. A computer node can have many connections

I have created two Java POJO classes but i'm not exactly sure about the annotations required. I have read the hibernate tutorial but i don't see explanation on class without a identifier(ie: Connection).

ComputerNode.java:

@Entity
@Table(name="ComputerNodes")
public class ComputerNode {

@Id
@Column(name="nodeid")
private int nodeId;

@Column(name="nodename")
private String nodeName;

@OneToMany
private Set<Connection> connections;

.... //getter and setters
}

Connection.java

//What annotation should i use since this class doesn't have identifier?
public class Connection {

@Column(name="nodeid")
private int nodeId;

@Column(name="connstatus")
private String connStatus;
}

What type of class is Connection supposed to be? @Embeddable? What should I do to create a one-to-many relationship between the two classes?

================

Update

public List<ComputerNode> getComputerNodes() {
    //the query to inner join is:
    return sessionFactory.getCurrentSession().createQuery("from ComputerNode as node inner join Connection as conn").list();
}

for (ComputerNode cn : getComputerNodes) {
 System.out.println(cn.getNodeId() + ',' + cn.getNodeName());

 for (Connection c : cn.getConnections) {
   System.out.println(c.getConnStatus());
 }
}

@Id is only for Entity class. But Connection can't be entity can it? — chrizonline, Nov 28 '14 at 08:28
Connection should be entity only. I think there should be foreign key for that column you use `@Id` in Connection Entity class. — Rookie007, Nov 28 '14 at 08:33

score 1 · Answer 1 · edited May 23 '17 at 12:05

1

Try this:

@Entity
@Table(name="ComputerNode")
public class ComputerNode {

@Id
@Column(name="nodeid")
private int nodeId;

@Column(name="nodename")
private String nodeName;

@OneToMany(mappedBy="computerNode")
private Set<Connection> connections;

.... //getter and setters
}

Connections:

@Entity
@Table(name="Connection")
public class Connection {

@ManyToOne
@JoinColumn(name="nodeid")
private ComputerNode computerNode;

...
}

If Connection table doesn't have any primary key, check this solution: Hibernate and table without PK

===========

Updated

If you want to select ComputerNode entities use this query: sessionFactory.getCurrentSession().createQuery("select node from ComputerNode as node").list();

edited May 23 '17 at 12:05

Community

1
1

answered Nov 28 '14 at 08:30

Pavel Parshin

507
4
15

i got an error: no identifier specified for entity: org...Connection. For a class to be an Entity, it must have @Id. – chrizonline Nov 28 '14 at 08:37
Do you have any primary key in table Connection? – Pavel Parshin Nov 28 '14 at 08:40
Connection table doesn't have a primary key. and i can't add one because im not allowed to modify the schema – chrizonline Nov 28 '14 at 08:47
ok i set @Id to multiple properties in Connection. But im getting Path expected for join – chrizonline Nov 28 '14 at 09:05
Joins use associations between entities. Check that Connection table is entity. Or update your code, please. – Pavel Parshin Nov 28 '14 at 09:16
Both the classes are @Entity annotated – chrizonline Nov 28 '14 at 09:30
You should use paths in join operations. Look at my updated answer and check [documentation](http://docs.jboss.org/hibernate/core/3.6/reference/en-US/html_single/#queryhql-joins) – Pavel Parshin Nov 28 '14 at 09:54
sigh. I'm getting unknown column 'connection1_.node.nodeid' in field list – chrizonline Nov 28 '14 at 10:59
okay i fixed the unknown column. now im getting Ljava.lang.object: cannot be cast to org....ComputerNode... – chrizonline Nov 28 '14 at 11:25
i updated my questions, showing how i query and extract the data – chrizonline Nov 28 '14 at 11:30
Updated my answer. Also don't forget for implement correct hashCode() and equals() in entities. – Pavel Parshin Nov 28 '14 at 12:46

Secondo · Answer 2 · 2014-11-28T08:59:57.337

0

You have to make the Connection class to become an entity also, for you to have a relationship between two entity class just put @Entityon top it

@Entity
@Table(name="Connection")

public class Connection {  
@ManyToOne()
//this will join the nodeId of ComputerNode entity class to Connection entity class
@JoinColumn(name="nodeId")
private ComputerNode nodeId;

@Column(name="connstatus")     
private String connStatus;

}

hope this will help you.

edited Nov 28 '14 at 08:59

answered Nov 28 '14 at 08:34

Secondo

451
4
9

nodeid isnt a primary key in Connection table while it is a primary key in ComputerNode table – chrizonline Nov 28 '14 at 08:45
i edited my answer please explain why down vote so that i can fix it tnx in advance – Secondo Nov 28 '14 at 09:00
now i see my answer is the same what @Pavel answer -_-. nice try for me :) – Secondo Nov 28 '14 at 09:03

score 0 · Answer 3 · answered Nov 28 '14 at 09:01

0

I'm not really sure what you have to do without id.

Maybe this can help...

@OneToMany (cascade = {CascadeType.ALL}) 
@JoinTable(
    name="Connection",
    joinColumns = @JoinColumn( name="nodeid")
)
private Set<Connection> connections;

best!

answered Nov 28 '14 at 09:01

Jordi Castilla

26,609
8
70
109

Hibernate - One to Many associations

Update

3 Answers3