Writing Evolutions

Evolution files describe a set of changes made to an app or its models. These are Python files that live in the appdir/evolutions/ directory. The name of the file (minus the .py extension) is called an evolution label, and can be whatever you want, so long as it’s unique for the app. These files look something like:

myapp/evolutions/my_evolution.py
from __future__ import unicode_literals

from django_evolution.mutations import AddField


MUTATIONS = [
    AddField('MyModel', 'my_field', models.CharField, max_length=100,
             null=True),
]

Evolution files can make use of any supported App and Model Mutations (classes like AddField above) to describe the changes made to your app or models.

Once you’ve written an evolution file, you’ll need to place its label in the app’s appdir/evolutions/__init__.py in a list called SEQUENCE. This specifies the order in which evolutions should be processed. These look something like:

myapp/evolutions/__init__.py
from __future__ import unicode_literals

SEQUENCE = [
    'my_evolution',
]

Example

Let’s go through an example, starting with a model.

blogs/models.py
class Author(models.Model):
    name = models.CharField(max_length=50)
    email = models.EmailField()
    date_of_birth = models.DateField()


class Entry(models.Model):
    headline = models.CharField(max_length=255)
    body_text = models.TextField()
    pub_date = models.DateTimeField()
    author = models.ForeignKey(Author)

At this point, we’ll assume that the project has been previously synced to the database using something like ./manage.py syncdb or ./manage.py migrate --run-syncdb. We will also assume that it does not make use of migrations.

Modifying Our Model

Perhaps we decide we don’t actually need the birthdate of the author. It’s just extra data we’re doing nothing with, and increases the maintenance burden. Let’s get rid of it.

 class Author(models.Model):
     name = models.CharField(max_length=50)
     email = models.EmailField()
-    date_of_birth = models.DateField()

The field is gone, but it’s still in the database. We need to generate an evolution to get rid of it.

We can get a good idea of what this should look like by running:

$ ./manage.py evolve --hint

Which gives us:

#----- Evolution for blogs
from __future__ import unicode_literals

from django_evolution.mutations import DeleteField


MUTATIONS = [
    DeleteField('Author', 'date_of_birth'),
]
#----------------------

Trial upgrade successful!

As you can see, we got some output showing us what the evolution file might look like to delete this field. We’re also told that this worked – this evolution was enough to update the database based on our changes. If we had something more complex (like adding a non-null field, requiring some sort of initial value), then we’d be told we still have changes to make.

Let’s dump this sample file in blogs/evolutions/remove_date_of_birth.py:

blogs/evolutions/remove_date_of_birth.py
from __future__ import unicode_literals

from django_evolution.mutations import DeleteField


MUTATIONS = [
    DeleteField('Author', 'date_of_birth'),
]

(Alternatively, we could have run ./manage.py evolve -w remove_date_of_birth, which would create this file for us, but let’s start off this way.)

Now we need to tell Django Evolution we want this in our evolution sequence:

blogs/evolutions/remove_date_of_birth.py
from __future__ import unicode_literals

SEQUENCE = [
    'remove_date_of_birth',
]

We’re done with the hard work! Time to apply the evolution:

$ ./manage.py evolve --execute

You have requested a database upgrade. This will alter tables and data
currently in the "default" database, and may result in IRREVERSABLE
DATA LOSS. Upgrades should be *thoroughly* reviewed and tested prior
to execution.

MAKE A BACKUP OF YOUR DATABASE BEFORE YOU CONTINUE!

Are you sure you want to execute the database upgrade?

Type "yes" to continue, or "no" to cancel: yes

This may take a while. Please be patient, and DO NOT cancel the
upgrade!

Applying database evolution for blogs...
The database upgrade was successful!

Tada! Now if you look at the columns for your blogs_author table, you’ll find that date_of_birth is gone.

You can make changes to your models as often as you need to. Add and delete the same field a dozen times across dozens of evolutions, if you like. Evolutions are automatically optimized before applied, resulting in the smallest set of changes needed to get your database updated.