How to do it...

The following steps will show you how to switch from a foreign key relation to a many-to-many relation, while preserving the already existing data:

Add a new many-to-many field, called categories, as follows:

# myproject/apps/ideas/models.py
from django.db import models
from django.conf import settings
from django.utils.translation import gettext_lazy as _

from myproject.apps.core.model_fields import (
    MultilingualCharField,
    MultilingualTextField,
)

class Idea(models.Model):
    title = MultilingualCharField(
        _("Title"),
        max_length=200,
    )
    content = MultilingualTextField(
        _("Content"),
    )
    category = models.ForeignKey(
        "categories.Category",
        verbose_name=_("Category"),
        blank=True,
        null=True,
        on_delete=models.SET_NULL,
        related_name="category_ideas",
    )
    categories = models.ManyToManyField(
        "categories.Category",
        verbose_name=_("Categories"),
        blank=True,
        related_name="ideas",
    )

    class Meta:
        verbose_name = _("Idea")
        verbose_name_plural = _("Ideas")

    def __str__(self):
        return self.title

Create and run a schema migration, in order to add the new relationship to the database, as shown in the following code snippet:

(env)$ python manage.py makemigrations ideas
(env)$ python manage.py migrate ideas

Create a data migration to copy the categories from the foreign key to the many-to-many field, as follows:

(env)$ python manage.py makemigrations --empty \
> --name=copy_categories ideas

Open the newly created migration file (0003_copy_categories.py), and define the forward migration instructions, as shown in the following code snippet:

# myproject/apps/ideas/migrations/0003_copy_categories.py
from django.db import migrations


def copy_categories(apps, schema_editor):
    Idea = apps.get_model("ideas", "Idea")
    for idea in Idea.objects.all():
        if idea.category:
            idea.categories.add(idea.category)


class Migration(migrations.Migration):

    dependencies = [
        ('ideas', '0002_idea_categories'),
    ]

    operations = [
        migrations.RunPython(copy_categories),
    ]

Run the new data migration, as follows:

(env)$ python manage.py migrate ideas

Delete the foreign key category field in the models.py file, leaving only the new categories many-to-many field, as follows:

# myproject/apps/ideas/models.py
from django.db import models
from django.conf import settings
from django.utils.translation import gettext_lazy as _

from myproject.apps.core.model_fields import (
    MultilingualCharField,
    MultilingualTextField,
)

class Idea(models.Model):
    title = MultilingualCharField(
        _("Title"),
        max_length=200,
    )
    content = MultilingualTextField(
        _("Content"),
    )

    categories = models.ManyToManyField(
        "categories.Category",
        verbose_name=_("Categories"),
        blank=True,
        related_name="ideas",
    )

    class Meta:
        verbose_name = _("Idea")
        verbose_name_plural = _("Ideas")

    def __str__(self):
        return self.title

Create and run a schema migration, in order to delete the Categories field from the database table, as follows:

(env)$ python manage.py makemigrations ideas
(env)$ python manage.py migrate ideas