Sample.save() fails when there are no fields

[dvnstrcklnd]

Sample.save() fails when the SampleType has no fields defined

test_sample = session.Sample.new(
    name="Test 1",
    description="",
    project="Test",
    sample_type=session.SampleType.find_by_name("Test Sample Type")
)
print(test_sample)
test_sample.save()

produces

<Sample id=None name=Test 1 sample_type={'rid': 3, 'id': 6, 'name': 'Test Sample Type', 'description': 'New sample type description', 'created_at': '2020-11-18T13:08:56.000-08:00', 'updated_at': '2020-11-18T13:08:56.000-08:00'} rid=4>
Traceback (most recent call last):
  File "test_sample_new.py", line 16, in <module>
    test_sample.save()
  File "/usr/local/lib/python3.8/site-packages/pydent/models/sample.py", line 174, in save
    self.create()
  File "/usr/local/lib/python3.8/site-packages/pydent/models/sample.py", line 139, in create
    self.is_savable(do_raise=True)
  File "/usr/local/lib/python3.8/site-packages/pydent/models/sample.py", line 151, in is_savable
    for k, v in self.properties.items():
  File "/usr/local/lib/python3.8/site-packages/pydent/models/sample.py", line 105, in properties
    return self._field_value_dictionary(lambda ft: ft.name, self._property_accessor)
  File "/usr/local/lib/python3.8/site-packages/pydent/models/field_value_mixins.py", line 255, in _field_value_dictionary
    for fv in self.field_values:
TypeError: 'NoneType' object is not iterable

but when I add a field Test to the SampleType and define it in the new sample:

test_sample = session.Sample.new(
    name="Test 1",
    description="",
    project="Test",
    sample_type=session.SampleType.find_by_name("Test Sample Type"),
    properties={"Test": "foo"}
)
print(test_sample)
test_sample.save()

then it works.

[bjkeller]

@cashmonger can you take a look at this?