symfony 1.4 and propel - foreign constraints fails saved relation - foreign-keys

Question

I am working in a Symfony 1.4 project with Propel 1.4.2.
I have 2 related tables. workshop and trainers which is a many to many relation mapped by a join table (workshop_trainers) which contains the workshop_id and the trainer_id).
In my Workshop Form I have a select box for adding the trainers to the workshop. The problem is when the workshop is new (Create) I get an error:
Cannot add or update a child row: a foreign key constraint fails
This happens because, when saving the workshop_trainers relation the workshop_id field is null. Isn´t Propel intelligent enough to know that there is a relation between the tables and save the base object first? What I am doing wrong?
My trainer list widget.
$this->widgetSchema['workshop_trainer_list'] = new sfWidgetFormChoice(array(
'choices' => $trainers,
'multiple' => true,
));
Thanks for your help.

Answer 1

This is not fixing the problem but that's the easiest practical solution to this problem:
In the form, simply deactivate the workshop_trainer_list field if the object is a new one (doesn't have an ID yet).
Something like:
if ($this->getObject()->isNew())
{
$this->offsetUnset('workshop_trainer_list'); // not sure of that method name
}
A better solution is to update the doSave method to have the ID first, something like this:
protected function doSave($con = null)
{
$isNew = $this->getObject()->isNew();
if (null === $con)
{
$con = $this->getConnection();
}
// retrieve the value of workshop_trainer_list here and remove it from the form
$trainers = ...
$this->offsetUnset('workshop_trainer_list');
// save without it
parent::doSave($con);
// add it back
$this->getObject()->set...
// save
$this->getObject()->save($con);
}