ForeignKey is represented by django.forms.ModelChoiceField, which is a ChoiceField whose choices are a model QuerySet. See the reference for ModelChoiceField.
So, provide a QuerySet to the field’s
queryset attribute. Depends on how your form is built. If you build an explicit form, you’ll have fields named directly.
form.rate.queryset = Rate.objects.filter(company_id=the_company.id)
If you take the default ModelForm object,
form.fields["rate"].queryset = ...
This is done explicitly in the view. No hacking around.