formfield_for_foreignkey difficulty

I have formfield_for_foreignkey working for a couple of fields in my
substance model but find a brick wall when I try to restrict choices in
an inline m2m foreign key.

The m2m relation is substance<-m2m->substance where any substance can be
an ingredient in a mixture (also a substance).

The through table is ...

class Substance_Ingredients(models.Model):
substance = models.ForeignKey('Substance', null=True, blank=True,
related_name='base_substance')
ingredient = models.ForeignKey('Substance', null=True, blank=True,)
proportion = models.DecimalField(null=True, blank=True,)

Is that a limitation in the admin or should I try harder?

Thanks

Mike

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