Emptying a table and filling with fixtures - django

Question

I'm working on a big project (tons of migrations already exist, etc...) and I'm at a point in which I must empty an entire table and then fill it again with ~10 elements. What is the Django approach to this situation? Fixtures? RunPython?

Answer 1

Deleting data from tables (Django)
A default Django-generated migration has nothing to do with populating your table. A default migration changes your database layout, e.g. creates or deletes tables, adds columns, changes parameters of columns etc. (Read until the end on how to use manual migrations to delete data from table!)
Deleting data once
What you want to do is delete entries in a table and not delete the whole table. Of course, you could remove the table from your models.py and then migrate which would delete the table (if no errors, read next) but that might result in unwanted behaviour and errors (e.g. other models have ForeignKeys to this table which would probably prevent you from deleting the table). You have two options:
Manually connect to database and run
DELETE * FROM your_table;
Use Python to do the job for you. You can open Django shell by executing python manage.py shell. Then you have to import your model and run .delete(). That would look like this:
$ python manage.py shell
# We are in Django Python shell now...
>> from app.models import Model_to_empty
>> Model_to_empty.objects.all().delete()
Deleting data from tables with manual migration files
If you want to create it as migration then you can write a migration file yourself. To make sure everything is smooth, run
python manage.py makemigrations
python manage.py migrate
first, to migrate any changes that could possible be done in between. Now, create your fake migration file like this:
If your last migration was number 0180, name your file something like 0181_manual_deletion_through_migration.py and put it in app/migrations where app is the app that contains the model that needs to be emptied and refilled.
You can use migrations.RunSQL class in your migrations which will execute statement given as argument while migrating.
Example migration file taken from one of my projects is:
from django.db import migrations, models
class Migration(migrations.Migration):
dependencies = [
('beer', '0001_initial'),
]
operations = [
migrations.AddField(
model_name='beer',
name='beer_type',
field=models.CharField(default=0, max_length=30),
preserve_default=False,
),
]
Let's break it down:
dependencies = [
('beer', '0001_initial'),
]
This describes the previous migration that altered the model. 'beer' is the name of the app, '0001_initial' is previous migration. Set this to name of model you want to delete entries from and the name of migration should be the last migration.
operations = [
migrations.AddField(
model_name='beer',
name='beer_type',
field=models.CharField(default=0, max_length=30),
preserve_default=False,
),
]
Inside of operations comes what needs to be done. In my example, it was adding a field, thus migrations.AddField. Remember I told you about migrations.RunSQL? Well, we can use it here like this:
operations = [
migrations.RunSQL("DELETE * FROM your_model;"),
# run SQL statements to populate your model again.
]
where instead of a comment you put the SQL statements that will populate the table with entries you want.
When you finish editing the fake migration file, just execute python manage.py migrate (NO python manage.py makemigrations!!).