@ManyToOne & @JoinColumn

[Musab Al-Rawi]

I am new to EJB and JPA and I need your help please.

Suppose that we have two Entities A (mapped to DB table A with Pk field named id)
and B (mapped to a DB table B with FK a_id that references A.id in a many to one relationship)

in entity class A I have:

@OneToMany (mappedby="bs")
private Set<B> Bees;

in entity class B I have:

@ManyToOne(name="a_id", referencedColumnName="id")
private A owner;

Based on my understanding that should work but Eclipse is giving me referenced column "id" in join column"a_id" cannot be resolved.
I checked eclipse's manual and it says that i need to make sure that the column exists in DB and it does, so why am I getting that error?

Your help is greatly appreciated,
Musab

[Mark Spritzler]

I think your topic says it all. I think you need an @JoinedColumn tag above the Collection. When I just have @ManyToOne and I let the tables be automatically created, then what happens if you have the parent and child tables, but it also creates a join table. When I add @JoinedColumn, it just creates parent and child tables where the child table gets a foreign key reference.

Mark

[Musab Al-Rawi]

Thanks Mark,
you gave me hope after I almost lost it

Tables are already created with FK constraint and everything in DB is set. Actually I am using the auto creation feature.

I am just trying to setup Many-to-one association between the two entities using @JoinColumn and it is not happening (gives the cannot be resolved error)

any idea?
thanks

[Sergio Tridente]

Have you tried the following?

@ManyToOne
@JoinColumn(name="a_id", referencedColumnName="id")
private A owner;

[Musab Al-Rawi]

Originally posted by Musab Al-Rawi:

in entity class B I have:

@ManyToOne(name="a_id", referencedColumnName="id")
private A owner;

I am sorry what I actually have is what Sergio suggested and that's what I am getting the error for.

So to recap this is what I have in entity class B (and it is causing an error)

@ManyToOne
@JoinColumn(name="a_id", referencedColumnName="id")
private A owner;

[ September 24, 2008: Message edited by: Musab Al-Rawi ]

[Sergio Tridente]

Can you post both classes' code in completion?

[Musab Al-Rawi]

Users table has primary key column named id
Groups table has a primary key column named id and a FK named user_id that references users.id


This is the One to many side of the relationship:

@Table(schema="test", name="users")
public class User implements Serializable {
@Id
@GeneratedValue
@Column
private String id;
 
@Column
private String email;
 
@Column
private String passwd;
 
@OneToMany(mappedBy="owner", cascade = REMOVE)
private Set<Group> ownedGroups;

@ManyToMany
@JoinTable(
name="UserGroups",
joinColumns=
@JoinColumn(name="user_id", referencedColumnName="id"),
inverseJoinColumns=
@JoinColumn(name="Group_id", referencedColumnName="id")
)
private Set<Group> belongsToGroups;

private static final long serialVersionUID = 1L;
 
public User() {
super();
}
 
public String getId() {
return this.id;
}
 
public void setId(String id) {
this.id = id;
}
 
public String getEmail() {
return this.email;
}
 
public void setEmail(String email) {
this.email = email;
}
 
public String getPasswd() {
return this.passwd;
}
 
public void setPasswd(String passwd) {
this.passwd = passwd;
}
 
public Set<Group> getOwnedGroups() {
return this.ownedGroups;
}
 
public void setOwnedGroups(Set<Group> groups) {
this.ownedGroups = groups;
}
 
public void setBelongsToGroups(Set<Group> belongToGroups) {
this.belongsToGroups = belongToGroups;
}
 
public Set<Group> getBelongsToGroups() {
return belongsToGroups;
}
}
 

This is the Many to one side
I haven't done the many to many relationship for this class yet:

@Entity
@Table(schema="test", name="groups")
public class Group implements Serializable {
@Id
private String id;
 
@Column
private String name;
 
@Column
private String description;
 
 
//this is where the problem is happening
@ManyToOne
@JoinColumn(name="user_id", referencedColumnName="id")
private User owner;


 
private static final long serialVersionUID = 1L;
 
public Group() {
super();
}
 
public String getId() {
return this.id;
}
 
public void setId(String id) {
this.id = id;
}
 
public String getName() {
return this.name;
}
 
public void setName(String name) {
this.name = name;
}
 
public String getDescription() {
return this.description;
}
 
public void setDescription(String description) {
this.description = description;
}
 
public User getOwner() {
return this.owner;
}
 
public void setOwner(User user) {
this.owner = user;
}
 
}
 
    

[ September 24, 2008: Message edited by: Musab Al-Rawi ]

[Sergio Tridente]

Hi Musab,

Try changing the User class to the following:

@Table(schema="test", name="users")
public class User implements Serializable {
@Id
private String id;
 
private String email;
 
private String passwd;
 
@OneToMany(mappedBy="owner", cascade = REMOVE)
private Set<Group> ownedGroups;

@ManyToMany
@JoinTable(
name="UserGroups",
joinColumns=
@JoinColumn(name="user_id", referencedColumnName="id"),
inverseJoinColumns=
@JoinColumn(name="Group_id", referencedColumnName="id")
)
private Set<Group> belongsToGroups;

private static final long serialVersionUID = 1L;
 
public User() {
super();
}
 
public String getId() {
return this.id;
}
 
public void setId(String id) {
this.id = id;
}
 
public String getEmail() {
return this.email;
}
 
public void setEmail(String email) {
this.email = email;
}
 
public String getPasswd() {
return this.passwd;
}
 
public void setPasswd(String passwd) {
this.passwd = passwd;
}
 
public Set<Group> getOwnedGroups() {
return this.ownedGroups;
}
 
public void setOwnedGroups(Set<Group> groups) {
this.ownedGroups = groups;
}
 
public void setBelongsToGroups(Set<Group> belongToGroups) {
this.belongsToGroups = belongToGroups;
}
 
public Set<Group> getBelongsToGroups() {
return belongsToGroups;
}
}

I don't think the @GeneratedValue annotation works on String attributes/fields. Also, you don't need to specify the @Column annotation if the column name is identical to the class' attribute/field name.

[Musab Al-Rawi]

Sergio thanks for the tips, I made the modifications and the problem is still there.

I noticed that this problem only happens with the USER table! The table is created and there is a column called id that acts as a PK.

[Musab Al-Rawi]

Anyone any suggestions please?