Development with Python

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$ scp :\
/home/elspeth/sites/superlists.ottg.eu/database/db.sqlite3 .
$ mv ../database/db.sqlite3 ../database/db.sqlite3.bak
$ mv db.sqlite3 ../database/db.sqlite3
Confirming the Error
We now have a local database that has not been migrated, and that contains some prob‐
lematic data. We should see an error if we try to run migrate :
$ python3 manage.py migrate --migrate
python3 manage.py migrate
Operations to perform:
[...]
Running migrations:
[...]
Applying lists.0005_list_item_unique_together...Traceback (most recent call
last):
[...]
return Database.Cursor.execute(self, query, params)
sqlite3.IntegrityError: columns list_id, text are not unique
Inserting a Data Migration
are a special type of migration that modifies data in the database rather
than changing the schema. We need to create one that will run before we apply the
integrity constraint, to preventively remove any duplicates. Here’s how we can do that:
$ git rm lists/migrations/0005_list_item_unique_together.py
$ python3 manage.py makemigrations lists --empty
Migrations for 'lists':
0005_auto_20140414_2325.py:
$ mv lists/migrations/0005_ lists/migrations/0005_remove_duplicates.py*
Check out for more info, but here’s how we add
some instructions to change existing data:
lists/migrations/0005_remove_duplicates.py.
# encoding: utf8
from django.db import models, migrations
def find_dupes(apps, schema_editor):
List = apps. get_model("lists", "List" )
for list_ in List.objects.all():
items = list_.item_set.all()
texts = set()
for ix, item in enumerate(items):
if item. text in texts:
item.text = '{} ({})'.format(item.text, ix)
item.save()
texts.add(item.text)
Inserting a Data Migration | 429