Re: How to represent a calendar month as a field in django models

On 2015-05-03 14:35, Matthys wrote:
> I posted the question also on stackoverflow:
>
> http://stackoverflow.com/questions/30017229/how-to-represent-month-as-field-on-django-model
>
> The question is for situations where a model instance relates to a
> specific month, but not to a specific day.

If the other suggestions don't work for you, you could store it as a
regular integer field for the number of months. A custom widget
would allow you to create an interface to manage it, and the value
would simply be "years * 12 + months - 1" and to extract the year and
month, you'd use something like

year, month = divmod(mymodel.my_yearmonth_field, 12)
month += 1

It would have the peculiar side-effect that internally the month
would be represented by a value 0..11 instead of 1..12

However, it has the advantage that it's only one field in the
database and compares naturally ("ORDER BY my_yearmonth_field")

You could even fancy it up something like the below.

-tkc

from django.db import models
from django.forms import widgets

class YearMonth(object):
def __init__(self, year, month):
self.year = year
self.month = month

class YearMonthFormField(widgets.Widget):
# for implementation ideas, see
# https://docs.djangoproject.com/en/1.8/_modules/django/forms/extras/widgets/#SelectDateWidget
pass

class YearMonthField(models.IntegerField):
description = "A field for managing year+month without dates"
def __init__(self, *args, **kwargs):
super(YearMonthField, self).__init__(*args, **kwargs)
def to_python(self, value):
if isinstance(value, YearMonth):
return value
if value is None:
return value
year, month = divmod(value, 12)
return YearMonth(year, month + 1)
def from_db_value(self, value, expression, connection, context):
return self.to_python(value)
def get_prep_value(self, value):
return value.year * 12 + value.month - 1
def formfield(self, **kwargs):
defaults = {'form_class': YearMonthFormField}
defaults.update(kwargs)
return super(YearMonthField, self).formfield(**defaults)

class MyModel(models.Model):
my_yearmonth_field = YearMonthField()

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