JPA Many to Many relationship with attribute to self

[Leonardo Shikida]

How do I map this relationship using JPA?

TIA

Leo

[Angel Taveras]

Hello, Leonardo, could you post please at least the basic mapping that you're trying to map.

Regards,

Angel

[Wouter Oet]

And please UseRealWords. It took me 2 minutes at google to find out that TIA it means Thanks In Advance.

[Leonardo Shikida]

Let´s suppose I am trying to map something like this

- One section may contain several subsections
- Each subsection may belong to any set of sections
- A subsection is also a section
- We must keep the order these subsections are stored (the user must know which one is the first, which one is the second, etc)
- There is no limit to the hierarchy level

So it seems I can have a table like this

Section <--N---1--SectionRelationship--1----N-->Section

where SectionRelationship contains an attribute, let´s say, "order" (integer)

How do I map this into JPA?

TIA

Leo

[Karsten Wutzke]

We can't do the work for you.

It looks like you need two tables, one for the m:n links and one for the nodes. -> You are effectively creating a graph structure in a DB. There's actually nothing special about that. The techniques around that are sufficiently documented.

Karsten

[Angel Taveras]

Leo, i think that this could be mapped as follows:

  
@Entity
public class Section
{
  ...

  @ManyToMany
  @JoinTable(name="SectionRelationShip", joinColumns = {@JoinColumn(name="section_id")}, inverseJoinColumns={@JoinColumn(name="subsection_id"})
  public List<Section> getSubSections() { return subSections; }

  ...
}

In JPA 1.0 you cannot sort the resulting List with an aditional column just with the columns provided in the target entity; with Hibernate annotation @OrderColumn(name="order") you could. I didn't understand when you write

- Each subsection may belong to any set of sections

that means a bidirectional relationship?

Regards,

Angel

[Leonardo Shikida]

Hi Karsten. Well, I am not cutting and pasting my requirements and asking people to do my job. Sorry if you´ve thought I was doing this.

From this site - http://en.wikibooks.org/wiki/Java_Persistence/ManyToMany#Mapping_a_Join_Table_with_Additional_Columns - there is an example, so it seems I can´t use @ManyToMany directly. Instead, I am using two 1:N relationships

The problem is that the relationship is a self join, so I was a little bit confused about keeping 2 Lists in the same object to acomodate both relationships (yes, bi-directional).

So a little example would be nice, that´s what I am asking for.

TIA

Leo

[James Sutherland]

If you use JPA 2.0 you can just use an OrderColumn.

i.e.

@Entity  
public class Section  
{  
  ...  
  
  @ManyToMany  
  @JoinTable(name="SectionRelationShip", joinColumns = {@JoinColumn(name="section_id")}, inverseJoinColumns={@JoinColumn(name="subsection_id"})  
  @OrderColumn("order_index");
  public List<Section> getSubSections() { return subSections; }  
  
  ...  
} 

See,
http://en.wikibooks.org/wiki/Java_Persistence/Relationships#Order_Column_.28JPA_2.0.29