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

[cowwoc <cowwoc@xxxxxxxxxxxxxxxx>]

BTW: Short-term, I would be quite happy to settle for a simple workaround
than waiting for a fix, though if the fix is as simple to implement as I
mentioned (dropping the primary key, adding an id) then I'd be even happier
if you could implement it in the near future :)

This issue is very pressing for me right now because I don't see a simple
workaround and my customer requires this feature (a single playlist to allow
duplicate videos).

Gili


cowwoc wrote:
> 
> 
> 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


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