Re: [eclipselink-users] @ManyToMany relationship for a List?

[cowwoc <cowwoc@xxxxxxxxxxxxxxxx>]

James Sutherland wrote:
> 
> technically it is possible on a ManyToMany or when using a join table, but
> still rare for most models to want this.  I'm not certain what the JPA
> spec infers on duplicate support, but I assume it is either not supported,
> or not required, or more likely unspecified.
> 

I think the spec implies it. Looking at sections 2.1.8.5.1 and 2.1.8.5.2
compare the contents of "The following mapping defaults apply" paragraphs.
You will notice that the last sentence of 2.1.8.5.1 requires a unique key
constraint whereas 2.1.8.5.2 does not. I am asking precisely for this:
@ManyToMany should *not* enforce a unique constraint on pair of foreign
keys. Removing the primary key should be enough to satisfy this requirement.


James Sutherland wrote:
> 
> The main issue for relational is that things become tricky, i.e. insertion
> may work, but if you remove one of the duplicates, you now need to delete
> them all, then insert n-1 back.
> 

I think you're making it more complicated than it needs to be. Instead of
having a table with two foreign keys all you'd need is: [id, key1, key2]
where id is the primary key. You then issue a query that returns the first
row containing the values you'd like to delete, and delete it by its id.


Gili


FYI, I filed a bug report for the original issue:
https://bugs.eclipse.org/bugs/show_bug.cgi?id=256978

Gili



Anyway, on to your second question. Implementing indexed lists. What
do you mean implementing indexed lists on top of JPA?

java.util.List is already indexed..so I don't really follow your question.


JPA 1.0 does not retain the element index. The best you can do is @OrderBy.
If the order is arbitrary (chosen by the end-user) you need to rely in
ordering by indexes but currently JPA will not manage indexes for you under
the hood. For example, if you want to insert an element between two other
elements you will need to update the indexes of all elements that allow,
etc.

Gili






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