I want an "active_in" attribute as a timeframe. I assume that the DBMS is optimized for the postgresql tsrange field, and as such it is preferable to utilize the DateTimeRangeField rather than 2 separate fields for start_date and end_date.
Doing this I desire a default value for the field.
active_in = models.DateTimeRangeField(default=timezone.now+'-'+timezone.now+10YEARS)
Is my assumption about the DateTimeRangeField performance true?
Is there a smart solution be it creating a new; function,class or
simply manipulating the 2nd last digit?
My possible solutions:
Code using string manipulation:
active_in = models.DateTimeRangeField(default=timezone.now+'-'+timezone.now[:-2]+'30')
Code using custom function object: (adjusted from here: https://stackoverflow.com/a/27491426/7458018)
def today_years_ahead():
return timezone.now + '-' timezone.now() + timezone.timedelta(years=10)
class MyModel(models.Model):
...
active_in = models.DateTimeRangeField(default=today_years_ahead)
There's no need for string manipulation, as the documented Python type for this field is DateTimeTZRange.
I can't say I've ever used this field before, but something like this should work:
from psycopg2.extras import DateTimeTZRange
from django.utils import timezone
from datetime import timedelta
def next_ten_years():
now = timezone.now()
# use a more accurate version of "10 years" if you need it
return DateTimeTZRange(now, now + timedelta(days=3652))
class MyModel(models.Model):
...
active_in = models.DateTimeRangeField(default=next_ten_years)
Related
I have 2 fields in a model I need that when I change the value of a field to calculate the other
Example:
Date_mission1_equipe=models.DateField (null=True,blank=True,max_length=200)
Date_mission2_equipe=models.DateField (null=True,blank=True,max_length=200)
for example if i choose 01/01/2019 for Date_mission1_equipe automatically Date_mission2_equipe should be 02/01/2019
There are few ways to handle this. You could override the model save method. Or perhaps the cleanest is to use a pre_save signal, as shown below.
from django.db.models.signals import pre_save
def change_date(sender, instance, **kwargs):
my_object = MyModel.objects.get(id=instance.id)
if instance.Date_mission1_equipe:
a.Date_mission2_equipe = ...
pre_save.connect(change_date, sender=MyModel)
It is often recommended not to store calculated values like this in the database. Instead, just perform the calcaultion when you will need it.
from datetime import timedelta
class Mission(models.Model):
date_mission1_equipe = models.DateField(null=True, blank=True)
#property
def date_mission2_equipe(self):
return self.date_mission1_equipe + timedelta(days=1)
You can now do something like this:
first_mission = Mission.objects.get(id=1)
first_mission.date_mission1_equipe
<01/01/2019>
first_mission.date_mission2_equipe
<02/01/2019>
That's just an example. The date would probably need to be formatted for your needs, and will not automatically output in the format I put above.
Also, you do not need max_length on a DateField. It makes no sense for this field type.
I have a Booking model that has start and end datetime fields. I want to know how many days a booking covers. I can do this in Python but I need this value for further annotations.
Here's what I've tried:
In [1]: Booking.objects.annotate(days=F('end')-F('start'))[0].days
Out[1]: datetime.timedelta(16, 50400)
There are a few problems here:
I want an integer (or other number type I can use in calculations) of days as the output, not a timedelta. Setting output_field doesn't do anything meaningful here.
My sums are based on datetimes. Subtractions like this, without removing the time could lead to the whole number of days being off.
In Python I would do (end.date() - start.date()).days + 1. How can I do that in-database, preferably through the ORM (eg database functions), but a RawSQL would suffice to get this out the door?
I've written a couple of database functions to cast and truncate the dates to solve both problems under PostgreSQL. The DATE_PART and DATE_TRUNC internal function I'm using are DB-specific ☹
from django.db.models import Func
class DiffDays(Func):
function = 'DATE_PART'
template = "%(function)s('day', %(expressions)s)"
class CastDate(Func):
function = 'date_trunc'
template = "%(function)s('day', %(expressions)s)"
Then I can:
In [25]: Booking.objects.annotate(days=DiffDays(CastDate(F('end'))-CastDate(F('start'))) + 1)[0].days
Out[25]: 18.0
There is another, easy solution of this problem. You can use:
from django.db.models import F
from django.db.models.functions import ExtractDay
and then:
Booking.objects.annotate(days=(ExtractDay(F('end')-F('start'))+1))[0].days
If you are using MYSQL database, You could do it using Custom DB Function as,
from django.db.models.functions import Func
class TimeStampDiff(Func):
class PrettyStringFormatting(dict):
def __missing__(self, key):
return '%(' + key + ')s'
def __init__(self, *expressions, **extra):
unit = extra.pop('unit', 'day')
self.template = self.template % self.PrettyStringFormatting({"unit": unit})
super().__init__(*expressions, **extra)
function = 'TIMESTAMPDIFF'
template = "%(function)s(%(unit)s, %(expressions)s)"
Usage
from django.db.models import F, IntegerField
booking_queryset = Booking.objects.annotate(
days=TimeStampDiff(F('start'), F('end'), output_field=IntegerField()))
if booking_queryset.exist():
print(booking_queryset[0].__dict__)
I am fairly new to django and I am trying to constrain a django model field such that the age less than 25 years is shown as an error (using the datefield). So, I have the following model:
dob = models.DateField(blank=False, )
I am wondering how one can apply the above constraint in a django model.
Thanks.
I've just come across the same problem, and here is my solution for a custom field validator that checks for a minimum age value:
from django.utils.deconstruct import deconstructible
from django.utils.translation import ugettext_lazy as _
from django.core.validators import BaseValidator
from datetime import date
def calculate_age(born):
today = date.today()
return today.year - born.year - \
((today.month, today.day) < (born.month, born.day))
#deconstructible
class MinAgeValidator(BaseValidator):
message = _("Age must be at least %(limit_value)d.")
code = 'min_age'
def compare(self, a, b):
return calculate_age(a) < b
The calculate_age snippet is from this post.
Usage:
class MyModel(models.Model):
date_of_birth = models.DateField(validators=[MinAgeValidator(18)])
You need to create custom field validator.
Unfortunately you will need to hardcode the age value inside validator function, since it doesn't allow you to pass any arguments.
Then to calculate age use this snippet to correctly cover leap years.
EDITED:
How can I set a Django field's default to a function that gets evaluated each time a new model object gets created?
I want to do something like the following, except that in this code, the code gets evaluated once and sets the default to the same date for each model object created, rather than evaluating the code each time a model object gets created:
from datetime import datetime, timedelta
class MyModel(models.Model):
# default to 1 day from now
my_date = models.DateTimeField(default=datetime.now() + timedelta(days=1))
ORIGINAL:
I want to create a default value for a function parameter such that it is dynamic and gets called and set each time the function is called. How can I do that? e.g.,
from datetime import datetime
def mydate(date=datetime.now()):
print date
mydate()
mydate() # prints the same thing as the previous call; but I want it to be a newer value
Specifically, I want to do it in Django, e.g.,
from datetime import datetime, timedelta
class MyModel(models.Model):
# default to 1 day from now
my_date = models.DateTimeField(default=datetime.now() + timedelta(days=1))
The question is misguided. When creating a model field in Django, you are not defining a function, so function default values are irrelevant:
from datetime import datetime, timedelta
class MyModel(models.Model):
# default to 1 day from now
my_date = models.DateTimeField(default=datetime.now() + timedelta(days=1))
This last line is not defining a function; it is invoking a function to create a field in the class.
In this case datetime.now() + timedelta(days=1) will be evaluated once, and stored as the default value.
PRE Django 1.7
Django [lets you pass a callable as the default][1], and it will invoke it each time, just as you want:
from datetime import datetime, timedelta
class MyModel(models.Model):
# default to 1 day from now
my_date = models.DateTimeField(default=lambda: datetime.now() + timedelta(days=1))
Django 1.7+
Please note that since Django 1.7, usage of lambda as default value is not recommended (c.f. #stvnw comment). The proper way to do this is to declare a function before the field and use it as a callable in default_value named arg:
from datetime import datetime, timedelta
# default to 1 day from now
def get_default_my_date():
return datetime.now() + timedelta(days=1)
class MyModel(models.Model):
my_date = models.DateTimeField(default=get_default_my_date)
More information in the #simanas answer below
[1]: https://docs.djangoproject.com/en/dev/ref/models/fields/#default
Doing this default=datetime.now()+timedelta(days=1) is absolutely wrong!
It gets evaluated when you start your instance of django. If you are under apache it will probably work, because on some configurations apache revokes your django application on every request, but still you can find you self some day looking through out your code and trying to figure out why this get calculated not as you expect.
The right way of doing this is to pass a callable object to default argument. It can be a datetime.today function or your custom function. Then it gets evaluated every time you request a new default value.
def get_deadline():
return datetime.today() + timedelta(days=20)
class Bill(models.Model):
name = models.CharField(max_length=50)
customer = models.ForeignKey(User, related_name='bills')
date = models.DateField(default=datetime.today)
deadline = models.DateField(default=get_deadline)
There's an important distinction between the following two DateTimeField constructors:
my_date = models.DateTimeField(auto_now=True)
my_date = models.DateTimeField(auto_now_add=True)
If you use auto_now_add=True in the constructor, the datetime referenced by my_date is "immutable" (only set once when the row is inserted to the table).
With auto_now=True, however, the datetime value will be updated every time the object is saved.
This was definitely a gotcha for me at one point. For reference, the docs are here:
https://docs.djangoproject.com/en/dev/ref/models/fields/#datetimefield
Sometimes you may need to access model data after creating a new user model.
Here is how I generate a token for each new user profile using the first 4 characters of their username:
from django.dispatch import receiver
class Profile(models.Model):
auth_token = models.CharField(max_length=13, default=None, null=True, blank=True)
#receiver(post_save, sender=User) # this is called after a User model is saved.
def create_user_profile(sender, instance, created, **kwargs):
if created: # only run the following if the profile is new
new_profile = Profile.objects.create(user=instance)
new_profile.create_auth_token()
new_profile.save()
def create_auth_token(self):
import random, string
auth = self.user.username[:4] # get first 4 characters in user name
self.auth_token = auth + ''.join(random.SystemRandom().choice(string.ascii_uppercase + string.digits + string.ascii_lowercase) for _ in range(random.randint(3, 5)))
You can't do that directly; the default value is evaluated when the function definition is evaluated. But there are two ways around it.
First, you can create (and then call) a new function each time.
Or, more simply, just use a special value to mark the default. For example:
from datetime import datetime
def mydate(date=None):
if date is None:
date = datetime.now()
print date
If None is a perfectly reasonable parameter value, and there's no other reasonable value you could use in its place, you can just create a new value that's definitely outside the domain of your function:
from datetime import datetime
class _MyDateDummyDefault(object):
pass
def mydate(date=_MyDateDummyDefault):
if date is _MyDateDummyDefault:
date = datetime.now()
print date
del _MyDateDummyDefault
In some rare cases, you're writing meta-code that really does need to be able to take absolutely anything, even, say, mydate.func_defaults[0]. In that case, you have to do something like this:
def mydate(*args, **kw):
if 'date' in kw:
date = kw['date']
elif len(args):
date = args[0]
else:
date = datetime.now()
print date
Pass the function in as a parameter instead of passing in the result of the function call.
That is, instead of this:
def myfunc(date=datetime.now()):
print date
Try this:
def myfunc(date=datetime.now):
print date()
This code:
import datetime
d_tomorrow = datetime.date.today() + datetime.timedelta(days=1)
class Model(models.Model):
...
timeout = models.DateTimeField(null=True, blank=True, default=d_tomorrow)
...
resuls in this error:
'datetime.date' object has no attribute 'date'
What am I doing wrong?
d_tomorrow is expected, by the Django ORM, to have a date attribute (apparently), but doesn't.
At any rate, you probably want to use a callable for the default date; otherwise, every model's default date will be "tomorrow" relative to the time the model class was initialized, not the time that the model is created. You might try this:
import datetime
def tomorrow():
return datetime.date.today() + datetime.timedelta(days=1)
class Model(models.Model):
timeout = models.DateTimeField(null=True, blank=True, default=tomorrow)
Problem solved:
from datetime import datetime, time, date, timedelta
def tomorrow():
d = date.today() + timedelta(days=1)
t = time(0, 0)
return datetime.combine(d, t)
models.DateTimeField expects the value to be datetime.datetime, not datetime.date
2015 Update:
Arrow makes this all much more straight forward.
Arrow is a Python library that offers a sensible, human-friendly approach to creating, manipulating, formatting and converting dates, times, and timestamps. It implements and updates the datetime type, plugging gaps in functionality, and provides an intelligent module API that supports many common creation scenarios. Simply put, it helps you work with dates and times with fewer imports and a lot less code.
Arrow is heavily inspired by moment.js and requests.
I had this problem when using the model from django.contrib.admin. I had two similar models, both with a date field (and both using auto_now_date=True - complete red herring); one worked, one had this error.
Turned out to be
def __unicode__(self):
return self.date
goes BANG, while this
def __unicode__(self):
return u'%s' % self.date
works just fine. Which is kind of obvious after the event, as usual.
This works for me:
import datetime
from datetime import timedelta
tomorrow = datetime.date.today() + timedelta(days=1)
class Test(models.Model):
timeout = models.DateTimeField(db_index=True, default=tomorrow)
Alternatively you could use tomorrow = datetime.datetime.now() + timedelta(days=1)
I tried out your code and it worked just fine. Can you verify that you are not modifying/redefining the import in some way?
Also try this:
import datetime as DT
d_tomorrow = DT.date.today() + DT.timedelta(days=1)
class Model(models.Model):
timeout = models.DateTimeField(null=True, blank=True, default=d_tomorrow)