Posted by: Zulfikar Akbar Muzakki | in Python | 3 years, 10 months ago | 0 comments

Working with Django, we must be familiar with makemigrations and the migrate command. We use makemigrations to automatically generate migration files, and migrate to apply them.

Problem

Recently I stumbled upon a problem where in short, I need to add created_at and updated_at (both are DateTimeField) to an existing table. Here’s the model for that table.

Class FarmerSample(models.Model):
    geom = models.PointField()
    farm = models.IntegerField()
    datum = models.DateField()
    sample_number = models.CharField(max_length=128)

    def __str__(self):
        return '{} | {}'.format(self.id, self.datum)

FarmerSample data has datum field, which is the date a record is associated with. It is not the date when a record was created, because they are most likely created or saved to the database a few days after the observations, which makes the the creation date more recent than the datum. I will not go into the detail as to why I need to add a field to indicate when a record was created and updated, but will focus only on my approach.

When we query the data, they look like:

(venv) kartoza@kartoza-thinkbook:~/mysite$ ./manage.py shell
Python 3.6.12 (default, Aug 17 2020, 23:45:20) 
[GCC 9.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
(InteractiveConsole)
>>> from app.models import FarmerSample
>>> FarmerSample.objects.all()
<QuerySet [<FarmerSample: 1 | 2019-11-06>, <FarmerSample: 2 | 2020-09-01>]>

Now that I need a field to indicate the timestamp when a record was created or updated, I just need to change the model to:

Class FarmerSample(models.Model):
    geom = models.PointField()
    farm = models.IntegerField()
    datum = models.DateField()
    sample_number = models.CharField(max_length=128)
    created_at = models.DateTimeField(auto_now_add=True)
    updated_at = models.DateTimeField(auto_now=True)

    def __str__(self):
        return '{} | {} | {} | {}'.format(self.id, self.datum, self.created_at, self.updated_at)

Setting auto_now=True to DateTimeField will let Django update its value when the object is saved, while auto_now_add=True in DateTimeField will let Django update its value when the object is created. With those in mind, auto_now_add=True works well to indicate when a record was created, while auto_now=True could tell us when was the last time a record was updated.

When we do a migration, Django will add a value for the new fields. And in our case, Django will ask us the default value when running makemigrations:

You are trying to add the field 'created_at' with 'auto_now_add=True' to farmersample without a default; the database needs something to populate existing rows.

 1) Provide a one-off default now (will be set on all existing rows)
 2) Quit, and let me add a default in models.py
Select an option:

I choose 1 and these prompts show:

Please enter the default value now, as valid Python
You can accept the default 'timezone.now' by pressing 'Enter' or you can provide another value.
The datetime and django.utils.timezone modules are available, so you can do e.g. timezone.now
Type 'exit' to exit this prompt
[default: timezone.now] >>>

I will just press enter, and those with auto_now_add and auto_now set as True will have the value of timezone.now(). While this is not what I want, at least now they have their respective values.

Now, I want the value to be based on its datum. Yes, I did not track when the existing data was created, but a FarmerSample data that has datum of 2017-07-01 could not have been created on 2021-01-31. So, I want the created_at to be 2 days after the datum.

Solutions

There are 2 possible solutions for this:

1. Create a command to fill created_at.

2. Make a custom migration.

I chose the latter, as creating a one-off Django command in the first option just to do such a simple thing is overkill and kind of make a bloated codebase. This is when RunPython comes into its own. You can check the documentation here.

My migration file currently looks like this.

# Generated by Django 2.2.14 on 2021-01-28 05:20
from django.db import migrations, models


class Migration(migrations.Migration):

    dependencies = [
        ('live_layer', '0020_auto_20201221_0433'),
    ]

    operations = [
        migrations.AddField(
            model_name='farmersample',
            name='created_at',
            field=models.DateTimeField(auto_now_add=True, default=django.utils.timezone.now),
            preserve_default=False,
        ),
        migrations.AddField(
            model_name='farmersample',
            name='updated_at',
            field=models.DateTimeField(auto_now=True),
        )
    ]

With RunPython, I could call some function when I run the migration file. First, I will create a function to set created_at to be 2 days after datum.

import pytz
import django.utils.timezone
from datetime import datetime, time, timedelta
from django.conf import settings


def set_created_at(apps, schema_editor):
    live_layer_db = schema_editor.connection.alias
    # Get the model
    FarmerSample = apps.get_model("live_layer", "FarmerSample")

    # Loop through all objects and set created_at
    for data in FarmerSample.objects.using(live_layer_db).all():
        # Datum is DateField, while created_at is DateTimeField with Timezone.
        # First, we need to get the datum value with timezone
        datum_with_tz = datetime.combine(data.datum, time(0, 0), tzinfo=pytz.timezone(settings.TIME_ZONE))

        # Set created_at to 2 days after datum.
        created_at = datum_with_tz + timedelta(2)
        data.created_at = created_at
        data.save()

apps and schema_editor are default parameters so RunPython can run this function.

Custom Migration Pitfall

One thing to note when using RunPython is that if we supply only a forward function, then the migration will not be reversible. A forward function is a function that will be called when applying a migration. To be able to unapply a custom migration file, we must also provide a reverse function, which is a funtion that will be called when unapplying the migration. In our case, our migration adds new fields and sets the value for them, so our reverse function does not need to do anything (like reverting the created_at value) because the fields will be removed anyway.

def unset_created_at(apps, schema_editor):
    # Our migrations added a new field, so when we unapply, those field will be deleted.
    # It's useless reverting the value of created_at when eventually the field itself is removed.
    pass

Then, we must call both forward and reverse function inside the migration’s operations.

migrations.RunPython(set_created_at, unset_created_at, atomic=True)

So now, our migration file looks like this.

# Generated by Django 2.2.14 on 2021-01-28 05:20

import pytz
import django.utils.timezone
from datetime import datetime, time, timedelta
from django.conf import settings
from django.db import migrations, models


def set_created_at(apps, schema_editor):
    live_layer_db = schema_editor.connection.alias
    # Get the model
    FarmerSample = apps.get_model("live_layer", "FarmerSample")

    # Loop through all objects and set created_at
    for data in FarmerSample.objects.using(live_layer_db).all():
        # Datum is DateField, while created_at is DateTimeField with Timezone.
        # First, we need to get the datum value with timezone
        datum_with_tz = datetime.combine(data.datum, time(0, 0), tzinfo=pytz.timezone(settings.TIME_ZONE))

        # Set created_at to 2 days after datum.
        created_at = datum_with_tz + timedelta(2)
        data.created_at = created_at
        data.save()


def unset_created_at(apps, schema_editor):
    # Our migratios added a new field, so when we unapply those field will be deleted.
    # It's useless reverting the value of created_at when eventually the field itself is removed.
    pass


class Migration(migrations.Migration):

    dependencies = [
        ('live_layer', '0020_auto_20201221_0433'),
    ]

    operations = [
        migrations.AddField(
            model_name='farmersample',
            name='created_at',
            field=models.DateTimeField(auto_now_add=True, default=django.utils.timezone.now),
            preserve_default=False,
        ),
        migrations.AddField(
            model_name='farmersample',
            name='updated_at',
            field=models.DateTimeField(auto_now=True),
        ),
        migrations.RunPython(set_created_at, unset_created_at, atomic=True)
    ]

Finally, run python manage.py migrate and check whether the created_at and updated_at has been updated. You must exit the current shell first, then re-enter the shell to check the updated data otherwise you will see old-formatted data.

(venv) kartoza@kartoza-thinkbook:~/mysite$ ./manage.py shell
Python 3.6.12 (default, Aug 17 2020, 23:45:20) 
[GCC 9.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
(InteractiveConsole)
>>> from app.models import FarmerSample
>>> FarmerSample.objects.all()
>>> FarmerSample.objects.all()
<QuerySet [<FarmerSample: 1 | 2019-11-06 | 2019-11-08 00:00:00+00:00 | 2021-01-29 02:59:56.747043+00:00>, <FarmerSample: 2 | 2020-09-01 | 2020-09-03 00:00:00+00:00 | 2021-01-29 02:59:56.747789+00:00>]>

According to what we defined in our FarmerSample's __str__, we can see that the created_at is already set to 2 days after the datum. updated_at on the other hand, is set to the datetime when we did the migrations, because that was when we updated the data with specific created_at value.

Pretty simple and straightforward, compared to using a one-off management command or simply updating the value from Django shell.

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