forms.py

class ServiceTypeForm(forms.ModelForm):
    class Meta:
        model = Service
        fields = ['name', 'service_type']

    def __init__(self, *args, **kwargs):
        self.project = kwargs.pop('project')
        super(ServiceTypeForm, self).__init__(*args, **kwargs)

    def save(self):
        service = super(ServiceTypeForm, self).save(commit=False)
        service.project = self.project
        service.save()
        return service

gistfile1.txt

{'service_type': [ValidationError([u'Select a valid choice. That choice is not one of the available choices.'])]}

tests.py

class ServiceTypeFormTest(TestCase):
    def setUp(self):
        self.project = Project.objects.create(name='google')
        self.service_type = ServiceType.objects.create(name='database', slug='database', template='{{ template }}')

    def test_form_save(self):
        form = ServiceTypeForm({
            'name': 'googledb',
            'service_type': self.service_type,
            }, project=self.project)
        self.assertTrue(form.is_valid())