Populate a select dropdown based on queryset

Hi,

I'm trying to populate a 'vaccine' dropdown based on a 'species' of a pet.  For example, if a pet is a 'dog' I want to display vaccines for a dog only.  If a pet is a cat, I want to display a list of vaccines for cats only.

How do I do this?  I googled, but didn't come up with anything solid.

Here is my models.py

class Pet(models.Model):
    user = models.ForeignKey(User, on_delete=models.CASCADE, null=True)
    name = models.CharField(max_length=100, null=True, db_index=True)
    species = models.CharField(choices=SPECIES_CHOICES, null=True, max_length=20)

    def __str__(self):
        return self.name


class Vaccine(models.Model):
    vacc_name = models.CharField(max_length=100)
    species = models.CharField(choices=SPECIES_CHOICES, null=True, max_length=20)
    pet = models.ManyToManyField(Pet, through='PetHasVaccine')

    def __str__(self):
        return self.vacc_name


class PetHasVaccine(models.Model):
    pet = models.ForeignKey(Pet, on_delete=models.DO_NOTHING)
    vaccine = models.ForeignKey(Vaccine, on_delete=models.DO_NOTHING)

    def __str__(self):
        return str(self.pet.name) + ' - ' + str(self.vaccine.vacc_name)

Here is my views.py

def add_vaccine(request, pet_pk):
    pet = Pet.objects.get(pk=pet_pk)
    if pet.user == request.user:
        form = VaccineForm(request.POST or None)
        if request.POST:
            if form.is_valid():
                return HttpResponse(form)
            else:
                return render(request, 'pet/vaccine_add.html', {'vacc_form': VaccineForm(request.POST), 'pet': pet})
        else:
            return render(request, 'pet/vaccine_add.html', {'vacc_form': form, 'pet': pet})


Here is my forms.py

class VaccineForm(ModelForm):

    class Meta:
        model = PetHasVaccine
        fields = '__all__'
        exclude = ('pet',)
        widgets = {
            'date_given': DateInput(),
            'date_due': DateInput(),
        }

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