Migrations in Django are auto-generated files from the models of an application that apply changes to a stricture of a Table whether it's the creation, the modification, or the deletion of a field.

There are some situations when you are adding a new field or doing some modifications that we'll require you to write custom migrations.🤔

Let's see.

Problem

We have an e-commerce application selling shoes and computers. (Weird example but that will do the work😂)

In the database, we have the two following tables.

from django.db import models


class Shoe(models.Model):
    name = models.CharField(max_length=100)
    brand = models.CharField(max_length=100)
    color = models.CharField(max_length=100)
    price = models.IntegerField()
    in_stock = models.BooleanField(default=True)
    description = models.TextField()
    image = models.ImageField(upload_to='images/')

    def __str__(self):
        return self.name

class Computer(models.Model):
    name = models.CharField(max_length=100)
    brand = models.CharField(max_length=100)
    color = models.CharField(max_length=100)
    price = models.IntegerField()
    in_stock = models.BooleanField(default=True)
    description = models.TextField()
    image = models.ImageField(upload_to='images/')

    def __str__(self):
        return self.name

We have the Computer table and the Shoe table with basically the same fields. A way to improve this and avoid DRY will be to create an AbstractModel class but well in the database that won't count that much.

Let's create a class called Product that will have the same fields and a product_type field to know the type of product, computer or shoe.

class Product(models.Model):

    PRODUCT_TYPE_CHOICES = (
        ('shoe', 'shoe'),
        ('computer', 'shoe'),
    )

    name = models.CharField(max_length=100)
    brand = models.CharField(max_length=100)
    color = models.CharField(max_length=100)
    price = models.IntegerField()
    in_stock = models.BooleanField(default=True)
    description = models.TextField()
    image = models.ImageField(upload_to='images/')
    product_type = models.CharField(max_length=100, choices=PRODUCT_TYPE_CHOICES)

    def __str__(self):
        return self.name

The Product class is created. Now what? You will probably run the python manage.py makemigrations and the python manage.py migrate command to apply the changes in the database.
The Product table will be created but we also want to have the Shoe and the Computer table data in this new table so we can get rid of them.
This will require writing custom migrations.

Supposing that we have created a separated Django application called product in the Django project, we can use the following command to generate a new empty migrations file in the product application.

python manage.py makemigrations product --empty

We'll have a file similar to this.

# Generated by Django 4.0.2 on 2022-04-03 10:36

from django.db import migrations


class Migration(migrations.Migration):

    dependencies = [
        ('product', '0001_initial'),
    ]

    operations = [
    ]

Great! We need now to import the Shoe and Computer models and write a function to migrate Shoe and Computer table data to Product table.

def migrate_to_product_model(apps, schema_editor):
    Product = apps.get_model('product', 'Product')
    Shoes = apps.get_model('shoe', 'Shoes')
    Computer = apps.get_model('computer', 'Computer')

    for shoe in Shoes.objects.all():
        Product.objects.create(
            name=shoe.name,
            brand=shoe.brand,
            color=shoe.color,
            in_stock=shoe.in_stock,
            description=shoe.description,
            image=shoe.image,
            product_type="shoe"
        )

    for computer in Computer.objects.all():
        Product.objects.create(
            name=computer.name,
            brand=computer.brand,
            color=computer.color,
            in_stock=computer.in_stock,
            description=computer.description,
            image=computer.image,
            product_type="computer"
        )

And here's the final migration code.

# Generated by Django 4.0.2 on 2022-04-03 10:36

from django.db import migrations

def migrate_to_product_model(apps, schema_editor):
    Product = apps.get_model('product', 'Product')
    Shoes = apps.get_model('shoe', 'Shoes')
    Computer = apps.get_model('computer', 'Computer')

    for shoe in Shoes.objects.all():
        Product.objects.create(
            name=shoe.name,
            brand=shoe.brand,
            color=shoe.color,
            in_stock=shoe.in_stock,
            description=shoe.description,
            image=shoe.image,
            product_type="shoe"
        )

    for computer in Computer.objects.all():
        Product.objects.create(
            name=computer.name,
            brand=computer.brand,
            color=computer.color,
            in_stock=computer.in_stock,
            description=computer.description,
            image=computer.image,
            product_type="computer"
        )


class Migration(migrations.Migration):

    dependencies = [
        ('product', '0001_initial'),
    ]

    operations = [
        migrations.RunPython(migrate_to_product_model)
    ]

This is an example of a custom migration. No need to worry about if the migrations fail and the database is messed up. Migrations transactions are atomic by default, then if something goes south, the changes won't be committed to the database.
🚀

For more reading, check the official documentation of migrations in Django.

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Writing Custom Migrations in Django

Problem