Weird date math bug?

[Lachlan Musicman]

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:

class Patient(models.Model):

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

[Lachlan Musicman]

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.

[James Schneider]

On Aug 31, 2017 9:44 PM, "Lachlan Musicman" <dat...@gmail.com> wrote:

Sorry, let me start again:

Yay, I'm not the only one that does it! Lol...

<snip>

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

<snip>

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.

I'm guessing that ChemoRegime.regime_age() contains line 95.

ChemoRegime defines the stop_date field with null=True and blank=True. I'm guessing this is because patients may not have yet stopped treatment, so they have no stop_date.

With that in mind, self.stop_date may be None in Python (and Null in the DB). Subtracting None from a real datetime.date object is not supported.

Ensure that stop_date has a date value before doing date math with it.

-James

[Lachlan Musicman]

On 1 September 2017 at 14:57, James Schneider <jrschn...@gmail.com> wrote:

I'm guessing that ChemoRegime.regime_age() contains line 95.

ChemoRegime defines the stop_date field with null=True and blank=True. I'm guessing this is because patients may not have yet stopped treatment, so they have no stop_date.

With that in mind, self.stop_date may be None in Python (and Null in the DB). Subtracting None from a real datetime.date object is not supported.

Ensure that stop_date has a date value before doing date math with it.

-James

I am so embarrassed. Thank you James, spot on.

cheers

L.

[James Schneider]

I am so embarrassed. Thank you James, spot on.

cheers

L.

No worries. I've stuck my foot in my mouth more than once on this list.

-James

[Melvyn Sopacua]

A few tips on your code:

def get_latest_chemo(self):
chemo = ChemoRegime.objects.filter(patient=self).latest('stop_date')

class ChemoRegime(models.Model):
patient = models.ForeignKey(Patient, on_delete=models.CASCADE)

Use the related manager here. To make it readable, use:

class ChemoRegime(models.Model):
patient = models.ForeignKey(Patient, on_delete=models.CASCADE,
related_name='chemo_regimes')

class Patient(models.Model):
def get_latest_chemo(self):
return self.chemo_regimes.latest('stop_date')

When providing properties for use in templates, make them properties -
makes the template more readable:

@property
def latest_chemo(self):
return self.chemo_regimes.latest('stop_date')

{{ patient.latest_chemo }}

Similarly you can abstract active regimes:

class ChemoRegime(models.Model):
@property
def is_active(self):
return self.stop_date is None

{% if chemo.is_active %}<span class="badge
badge-primary">Current</span>{% endif %}


Of course, latest_chemo, should really be this if
active and non-active regimes can be mixed:

@property
def has_active_chemo(self):
return self.chemo_regimes.filter(stop_date__isnull=True).count() > 0

@property
def latest_chemo(self):
if not self.chemo_regimes.count():
return None
if self.has_active_chemo:
return self.chemo_regimes.latest('start_date')
return self.chemo_regimes.latest('stop_date')

Hope this helps :)

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[James Schneider]

Also, if you are developing this application for use within the USA, be sure that you are not capturing or storing data that would fall under the HIPAA definition of personal medical data. If it does, your legal compliance responsibilities and liabilities will skyrocket.

I'm not a lawyer, so I can't say for sure whether or not these data fields would be considered under HIPAA, but a compromise of medical data is extremely expensive, and I believe you can be held personally liable in some cases, even if developing for an employer.

https://www.hhs.gov/hipaa/for-professionals/index.html

If not in the US, there may be other regulations.

-James

[Lachlan Musicman]

We are not in the US, but our laws are equally as strict. The culture isn't quite so litigious either.

Having said that, all "patients" will only have an id, and the id->name will be mapped on paper in a filing cabinet. On top of that, it's within the corporate firewall. Which isn't perfect, I'm sure, but it pretty good. You need to have access to the staff LAN (via eth) to access the site.

This is the first in a long "project to replace all access 2003/2007/2010 dbs" with something more...contemporary.

cheers

L.

 

[Lachlan Musicman]

Melvyn - have only just got around to implementing your suggestions. Excellent, worked a treat. Thank you for the new tricks!

Cheers