Ponytech

Initial set up

Let's say you are starting a pizzeria and want your first pizzas to be very simple, they'll have only one topping:

from django.db import models

class Topping(models.Model):
    name = models.CharField(max_length=20)

class Pizza(models.Model):
    name = models.CharField(max_length=20)
    topping = models.ForeignKey(Topping)

Create the initial migration file (assuming our app is called pizza) and run it:

$ ./manage.py makemigrations pizza
$ ./manage.py migrate


Migrations for 'pizza':
  0001_initial.py:
    - Create model Pizza
    - Create model Topping
    - Add field topping to pizza

And let's create a few pizzas to test our example:

$ ./manage.py shell

from pizza.models import Topping, Pizza
for topping_name, pizza_name in [('mozzarella','Margherita'), ('mushrooms', 'Capricciosa'), ('artichoke', 'Vegetariana'), ('pineapple', 'Hawaii')]:
    topping = Topping.objects.create(name=topping_name)
    pizza = Pizza.objects.create(name=pizza_name, topping=topping)

Here are our data:

sqlite> select * from pizza_topping;
1|mozzarella
2|mushrooms
3|artichoke
4|pineapple

sqlite> select * from pizza_pizza;
1|Margherita|1
2|Capricciosa|2
3|Vegetariana|3
4|Hawaii|4

Convert to Many to Many

After making a few pizzas you soon realize only one topping per pizza is not tasty enough. Let's tweak our model and add a field which have many toppings:

class Pizza(models.Model):
    name = models.CharField(max_length=20)
    topping = models.ForeignKey(Topping, related_name='old_topping')
    toppings = models.ManyToManyField(Topping)

We must add a related_name attribute on one of the 2 fields to have the reverse accessor (from Topping to Pizza) to work. We now run an automatic migration to create the intermediary table that will hold the new relation:

$ ./manage.py makemigrations pizza

Migrations for 'pizza':
  0002_auto_20150612_0903.py:
    - Add field toppings to pizza
    - Alter field topping on pizza

Great. If we run migrate we'll now have an empty table pizza_pizza_toppings that is able to store many toppings per pizza. But we want to keep toppings we already have on our pizzas. So let's create a data migration:

$ ./manage.py makemigrations pizza --empty

Migrations for 'pizza':
  0003_auto_20150612_0920.py:

Edit the newly created file pizza/migrations/0003_auto_xxx.py. We'll create a function that for each pizza, copy data from the field topping to the new field toppings:

from django.db import models, migrations

def forward(apps, schema_editor):
    Pizza = apps.get_model("pizza", "Pizza")
    for pizza in Pizza.objects.all():
        pizza.toppings.add(pizza.topping)

class Migration(migrations.Migration):

    dependencies = [
        ('pizza', '0002_auto_20150612_0903'),
    ]

    operations = [
        migrations.RunPython(forward)
]

After running migrate, we can see toppings relationships had been copied to the new table:

sqlite> select * from pizza_pizza_toppings;
1|1|1
2|2|2
3|3|3
4|4|4

Last step is to clean up our model and remove the old topping field:

class Pizza(models.Model):
    name = models.CharField(max_length=20)
    toppings = models.ManyToManyField(Topping)

$ ./manage.py makemigrations pizza

Migrations for 'pizza':
  0004_remove_pizza_topping.py:
    - Remove field topping from pizza

Voila! You can now start making delicious pizza!