django - ManyToManyField の選択肢を動的な外部モデルに制限する

Question

class TrialRun(models.Model):
    product = models.ForeignKey(NewProduct, verbose_name='Product',)

class Component(models.Model):
    product = models.ForeignKey(NewProduct, verbose_name='Product',)

class TrialIssue(models.Model):
    trialrun = models.ForeignKey(TrialRun, verbose_name='Trial Run',)
    components = models.ManyToManyField(
        Component,limit_choices_to{product=trialrun.product})

の考え方をどのように実現でき(Component, limit_choices_to={product=trialrun.product})ますか？

次は使えない

def __init__(self, *args, **kwargs):
    super(TrialIssueForm, self).__init__(*args, **kwargs)
    # make sure we're editing a saved model
    if self.instance.id:
        self.fields['components'].queryset = Component.objects.filter(product=self.instance.trialrun.product)