Re: Weird date math bug?

Sorry, let me start again:


Is there a weird date math bug in Django 1.11.4? I say weird because surely not?

I can't see any tickets in the tracker, but I'm definitely seeing it in production.

Using postgresql:


models.py

class ChemoType(models.Model):
    name = models.CharField(max_length=50, unique=True)

class Patient(models.Model):
    chemo_regimes = models.ManyToManyField(
        ChemoType,
        through='ChemoRegime',
    )  
    def get_latest_chemo(self):
        chemo = ChemoRegime.objects.filter(patient=self).latest('stop_date')
        return chemo

    def get_latest_chemo_age(self):
        chemo = ChemoRegime.objects.filter(patient=self).latest('stop_date')
        return chemo.regime_age()

class ChemoRegime(models.Model):
    patient = models.ForeignKey(Patient, on_delete=models.CASCADE)
    chemotype = models.ForeignKey(ChemoType, on_delete=models.CASCADE)
    start_date = models.DateField(null=True, blank=True)
    stop_date = models.DateField(null=True, blank=True)

    def regime_age(self):
        age = timezone.now().date() - self.stop_date
        return age.days




In django_extension's shell plus:

>>> p = Patient.objects.get(id=1)
>>> p.get_latest_chemo()
<ChemoRegime: Gemcitabine>
>>> p.get_latest_chemo_age()
12
>>> p.get_latest_chemo_age
<bound method Patient.get_latest_chemo_age of <Patient: test-1>>
>>> type(p.get_latest_chemo_age())
<class 'int'>




In the template

          {% for patient in object_list %}
            <tr>
                   <td><a href="{{ patient.get_absolute_url }}">{{ patient }}</a></td>
                   <td>{{ patient.get_latest_chemo_age }} ago</td>
            </tr>




But in the browser:

Exception Type:     TypeError
Exception Value:    

unsupported operand type(s) for -: 'datetime.date' and 'NoneType'

Exception Location:     ./patients/models.py in regime_age, line 95

line 95 is         age = timezone.now().date() - self.stop_date

What am I doing wrong?

cheers

L.

------
"The antidote to apocalypticism is apocalyptic civics. Apocalyptic civics is the insistence that we cannot ignore the truth, nor should we panic about it. It is a shared consciousness that our institutions have failed and our ecosystem is collapsing, yet we are still here — and we are creative agents who can shape our destinies. Apocalyptic civics is the conviction that the only way out is through, and the only way through is together. "

Greg Bloom @greggish https://twitter.com/greggish/status/873177525903609857

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