Using DurationField to store working hours - django-models

Question

I'm working on a small project where users can track their time spent working on a contract. Each contract has a defined required work time, that the user may work per month.
Now several question arise: How do I store this work time inside my Django model? I ended up using the DurationField from Django 1.8, but this comes with it's own problems, as described below. Should I maybe switch to an IntegerField and store the work time as minutes and convert it to the correct format inside the template? Then I'd need to re-convert it after a form was sent by the user to store it in the right form again. How and where (models.py, forms.py ..?) would I end up doing those two conversions?
When using the DurationField I come up with two big problems:
It always assumes a format of "hh:mm:ss", while I don't need any seconds for a work time definition. So my JavaScript TimePicker doesn't let me pick seconds and leaves them zeroed. This isn't the most beautiful solution I think.
When specifying a contract with more than 24 hours of work time (say 80/month) Django saves the DurationField value as "3 days, 8 hours", but I want it to show up as "80:00" inside my input field. I know this is the normal Python timedelta behaviour, but is there a way to customize it? At least just for the front-end user.
So my basic two questions are: should I stick with DurationField and somehow solve the problems I face or should I switch to some other field like IntegerField and do the conversions on my own, which I'm not sure where to start.

Answer 1

After putting this issue aside for quite some time, I came up with a solution to this after following this blog post: http://charlesleifer.com/blog/writing-custom-field-django/
So far the code works as I would like it to do. It stores the work time as an integer inside the database and displays it as HH:MM to the user.
Still I'm not sure if I did it correctly or if something is missing or maybe wrong in some special case? I can't wrap my head around the difference behind to_python and from_db_value. Also I removed value_to_string from the original code (see blog post) as it didn't seem to do anything.
class WorkingHoursFieldForm(CharField):
"""
Implementation of a CharField to handle validation of data from WorkingHoursField.
"""
def __init__(self, *args, **kwargs):
kwargs['max_length'] = 5
super(WorkingHoursFieldForm, self).__init__(*args, **kwargs)
def clean(self, value):
value = super(CharField, self).clean(value)
# Split submitted duration into list
hour_value = value.split('.')
# If list does not have two values, let us do some more validation
if len(hour_value) != 2:
# In this case the user did not supply the form with the correct format.
# Therefore we are going to assume that he does not care about
# the minutes and we will just append those for him!
if len(hour_value)<2:
value = hour_value[0] + ".00"
# This case should only arise when the format was not correct at all.
else:
raise ValidationError(_('Working hours entered must be in format HH.MM'))
# If the value is in the correct format, check if the total working hours
# exceed 80 hours per month (this equals 288.000 seconds)
if len(hour_value) == 2:
hours, minutes = map(int, value.split('.'))
total_seconds = hours*3600 + minutes*60
if total_seconds > 80 * 3600:
raise ValidationError(_('Contracts may not be longer than 80 hours!'))
return value
class WorkingHoursField(IntegerField):
"""
Creates a custom field so we can store our working hours in contracts.
Working hours are stored as an integer in minutes inside the database.
This field accepts input in the format HH.MM and will display it the same way.
"""
# Get values from database and return them as HH.MM
def from_db_value(self, value, expression, connection, context):
if value is None:
return value
hours, minutes = divmod(value, 60)
return "%02d.%02d" % (hours, minutes)
def to_python(self, value):
if value is None:
return value
if isinstance(value, (int, long)):
return value
# Split into two values and return the duration in minutes!
if isinstance(value, basestring):
hours, minutes = map(int, value.split('.'))
return (hours * 60) + minutes
# I do not know if this is really relevant here?
elif not isinstance(value, datetime.timedelta):
raise ValidationError('Unable to convert %s to timedelta.' % value)
return value
def get_db_prep_value(self, value, connection, prepared):
return value
# This is somehow needed, as otherwise the form will not work correctly!
def formfield(self, form_class=WorkingHoursFieldForm, **kwargs):
defaults = {'help_text': _('Please specify your working hours in the format HH:MM \
(eg. 12:15 - meaning 12 hours and 15 minutes)')}
defaults.update(kwargs)
return form_class(**defaults)