Association on a non primary key field - fix the commit order

We are trying to create an association which referenced column is not a primary key.

The source entity mapping looks like:

<entity name="Joiz\HomeBundle\Entity\ShowInstanceTeaser" table="teaser">
<many-to-one field="showInstance" target-entity="Joiz\ShowBundle\Entity\ShowInstance">
<join-column name="show_instance_extid" referenced-column-name="extid" />
</many-to-one>
</entity>

The destination entity mapping is:

<entity name="Joiz\ShowBundle\Entity\ShowInstance" table="show_instance" repository-class="Joiz\ShowBundle\Entity\ShowInstanceRepository">

<id name="id" column="id" type="integer">
<generator strategy="IDENTITY" />
</id>

<field name="extid" column="extid" type="string" unique="true" />

<!-- ... snip ... -->

</entity>

Doctrine will correctly create the tables with the above mapping but the association does not seem to work.

When we try to load some data in the tables:

$showInst = new ShowInstance();
$showInst->setExtid(999);
$manager->persist($showInst);

$teaser = new ShowInstanceTeaser();
$teaser->setShowInstance($showInst);
$manager->persist($teaser);

This will leave the column "teaser.show_instance_extid" null.

Doing the same but using the primary key for the association will work.

In a discussion on the #doctrine-dev channel on freenode beberlei explained:

<beberlei> ah the problem is probably the CommitOrderCalculator does not know of this foreign key
<beberlei> and sorts the inserts in the wrong order

<beberlei> you cannot add foreign keys say through some mapping information in annotations or xml the like
<beberlei> you have to create them manually
<beberlei> and in this case doctrine might end up in troubles depending on the commit order
<beberlei> hibernate solves this by allowing to specifiy additional constraints that are recognized while calculating the commit order, but this is not yet in tdoctrine2. can you open up a ticket for this?