Django auto now and auto now add

Question

For Django 1 1   I have this in my models py   class User models Model       created   models DateTimeField auto now add True      modified   models DateTimeField auto now True    When updating a row I get    Sun Nov 15 02 18 12 2009   error   home ptarjan projects twitter-meme django db backends mysql base py 84  Warning  Column  created  cannot be null  Sun Nov 15 02 18 12 2009   error    return self cursor execute query  args    The relevant part of my database is      created  datetime NOT NULL     modified  datetime NOT NULL    Is this cause for concern   Side question  in my admin tool  those two fields aren t showing up  Is that expected

User · Answer

Here s the answer if you re using south and you want to default to the date you add the field to the database   Choose option 2 then  datetime datetime now    Looks like this       manage py schemamigration myapp --auto    The field  User created date  does not have a default specified  yet is NOT NULL     Since you are adding this field  you MUST specify a default    value to use for existing rows  Would you like to      1  Quit now  and add a default to the field in models py     2  Specify a one-off value to use for existing columns now    Please select a choice  2    Please enter Python code for your one-off default value     The datetime module is available  so you can do e g  datetime date today     gt  gt  gt  datetime datetime now      Added field created date on myapp User

User · Answer

You can use timezone now   for created and auto now for modified   from django utils import timezone class User models Model       created   models DateTimeField default timezone now        modified   models DateTimeField auto now True    If you are using a custom primary key instead of the default auto- increment int  auto now add will lead to a bug   Here is the code of Django s default DateTimeField pre save withauto now and auto now add   def pre save self  model instance  add       if self auto now or  self auto now add and add           value   timezone now           setattr model instance  self attname  value          return value     else          return super DateTimeField  self  pre save model instance  add    I am not sure what the parameter add is  I hope it will some thing like   add   True if getattr model instance   id   else False      The new record will not have attr id  so getattr model instance   id   will return False will lead to not setting any value in the field

User · Answer

If you alter your model class like this   class MyModel models Model       time   models DateTimeField auto now add True      time editable   True   Then this field will show up in my admin change page

User · Answer

Talking about a side question  if you want to see this fields in admin  though  you won t be able to edit it   you can add readonly fields to your admin class   class SomeAdmin ModelAdmin       readonly fields     created   modified      Well  this applies only to latest Django versions  I believe  1 3 and above

User · Answer

But I wanted to point out that the opinion expressed in the accepted answer is somewhat outdated  According to more recent discussions  django bugs  7634 and  12785   auto now and auto now add are not going anywhere  and even if you go to the original discussion  you ll find strong arguments against the RY  as in DRY  in custom save methods   A better solution has been offered  custom field types   but didn t gain enough momentum to make it into django  You can write your own in three lines  it s Jacob Kaplan-Moss  suggestion    from django db import models from django utils import timezone   class AutoDateTimeField models DateTimeField       def pre save self  model instance  add           return timezone now     usage created at   models DateField default timezone now  updated at   models AutoDateTimeField default timezone now

User · Answer

class Feedback models Model      feedback   models CharField max length 100     created   models DateTimeField auto now add True     updated   models DateTimeField auto now True    Here  we have created and updated columns that will have a timestamp when created  and when someone modified feedback   auto now add will set time when an instance is created whereas auto now will set time when someone modified his feedback

User · Answer

Based on what I ve read and my experience with Django so far  auto now add is buggy   I agree with jthanism --- override the normal save method it s clean and you know what s hapenning   Now  to make it dry  create an abstract model called TimeStamped   from django utils import timezone  class TimeStamped models Model       creation date   models DateTimeField editable False      last modified   models DateTimeField editable False       def save self   args    kwargs           if not self creation date              self creation date   timezone now            self last modified   timezone now           return super TimeStamped  self  save  args    kwargs       class Meta          abstract   True   And then  when you want a model that has this time-stampy behavior  just subclass   MyNewTimeStampyModel TimeStamped       field1         If you want the fields to show up in admin  then just remove the editable False option

User · Answer

Is this cause for concern    No  Django automatically adds it for you while saving the models  so  it is expected      Side question  in my admin tool  those 2 fields aren t showing up  Is that expected    Since these fields are auto added  they are not shown   To add to the above  as synack said  there has been a debate on the django mailing list to remove this  because  it is  not designed well  and is  a hack      Writing a custom save   on each of my models is much more pain than using the auto now   Obviously you don t have to write it to every model  You can write it to one model and inherit others from it   But  as auto add and auto now add are there  I would use them rather than trying to write a method myself

User · Answer

auto now True didn t work for me in Django 1 4 1  but the below code saved me  It s for timezone aware datetime   from django utils timezone import get current timezone from datetime import datetime  class EntryVote models Model       voted on   models DateTimeField auto now True       def save self   args    kwargs           self voted on   datetime now   replace tzinfo get current timezone            super EntryVote  self  save  args    kwargs

User · Answer

I needed something similar today at work  Default value to be timezone now    but editable both in admin and class views inheriting from FormMixin  so for created in my models py the following code fulfilled those requirements   from   future   import unicode literals import datetime  from django db import models from django utils functional import lazy from django utils timezone import localtime  now  def get timezone aware now date        return localtime now    date    class TestDate models Model       created   models DateField default lazy          get timezone aware now date  datetime date            For DateTimeField  I guess remove the  date   from the function and change datetime date to datetime datetime or better timezone datetime  I haven t tried it with DateTime  only with Date

User · Answer

As for your Admin display  see this answer   Note  auto now and auto now add are set to editable False by default  which is why this applies

User · Answer

Any field with the auto now attribute set will also inherit editable False and therefore will not show up in the admin panel  There has been talk in the past about making the auto now and auto now add arguments go away  and although they still exist  I feel you re better off just using a custom save   method   So  to make this work properly  I would recommend not using auto now or auto now add and instead  define your own save   method to make sure that created is only updated if id is not set  such as when the item is first created   and have it update modified every time the item is saved   I have done the exact same thing with other projects I have written using Django  and so your save   would look like this   from django utils import timezone  class User models Model       created       models DateTimeField editable False      modified      models DateTimeField        def save self   args    kwargs               On save  update timestamps             if not self id              self created   timezone now           self modified   timezone now           return super User  self  save  args    kwargs    Hope this helps   Edit in response to comments   The reason why I just stick with overloading save   vs  relying on these field arguments is two-fold    The aforementioned ups and downs with their reliability   These arguments are heavily reliant on the way each type of database that Django knows how to interact with treats a date time stamp field  and seems to break and or change between every release   Which I believe is the impetus behind the call to have them removed altogether   The fact that they only work on DateField  DateTimeField  and TimeField  and by using this technique you are able to automatically populate any field type every time an item is saved  Use django utils timezone now   vs  datetime datetime now    because it will return a TZ-aware or naive datetime datetime object depending on settings USE TZ    To address why the OP saw the error  I don t know exactly  but it looks like created isn t even being populated at all  despite having auto now add True   To me it stands out as a bug  and underscores item  1 in my little list above   auto now and auto now add are flaky at best

User · Answer

I think the easiest  and maybe most elegant  solution here is to leverage the fact that you can set default to a callable  So  to get around admin s special handling of auto now  you can just declare the field like so  from django utils import timezone date field   models DateField default timezone now   It s important that you don t use timezone now   as the default value wouldn t update  i e   default gets set only when the code is loaded   If you find yourself doing this a lot  you could create a custom field  However  this is pretty DRY already I think

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