Fix Django Migration : pkey errors

Touching the same models at the same time your peers do can cause frustration on migrations. Yeah, many problems could occur, so let’s see one of the ways to deal with one of those.

It could happen when you switch between branches which touch the same models or trying to create models at the same time. Making migrations script would work fine ( python manage.py makemigrations). But when you execute migration using python manage.py migrate? Can’t tell… Occasionally, it will raise the pkey errors series. Somethings like:

“django_migrations _pkey” DETAIL: Key (id)=( ) already exists

It is saying that the primary key of the django_migration already exists. So the attempt to run the migration fails.

The error message could be different for different database. If you use PostgreSQL, the error message will look like

django.db.utils.IntegrityError: duplicate key value violates unique constraint “django_migrations_pkey”
DETAIL: Key (id)=(SomeNumber) already exists.

“django_migrations” is a table that Django keep historical data of what has been migrated and when.
Let’s take a look…

This picture just shows the tables, captured from PgAdmin

fields in django_migrations table (captured from pgAdmin)

The picture above shows fields ofdjango_migrations . It has appliedfield which keeps the timestamp. Also, save app and migration script name.
Now, id fields, which is auto-generated primary key here. The problem is the sequence, which keep track of the latest primary key isn’t up-to-date. Let’s explain this by giving an example,

Given that django_migrations has 20 items, the lastest one has pkey=25. But the sequence of django_migrations table is 22. It will cause error we mentioned above.

To fix it, I propose 2 ways.

1 ) The neat way,
Update the sequence. You can make things right by running SQL command at the database.

Update sequence at your database

This will alter sequence of django_migrations table. You can now successfully run your migration script again.

( O v O )

2 ) The lazy way to fix it is just repeating the python manage.py migrate. Notice that each time you run it the pkey in error message increases. The sequence is updated every time the script fails. You rerun it until it runs successfully.

django_migrations pkey issue fixed, you might get more errors from django_content_type and auth_permission tables.

When you add a new model to the apps, Django keeps track of all available models incontent_type tables. And also store its auth_permission accordingly. You can just apply the same technique to fix the sequence here.

django.db.utils.IntegrityError: duplicate key value violates unique constraint “django_content_type_pkey”
DETAIL: Key (id)=( ) already exists.

django.db.utils.IntegrityError: duplicate key value violates unique constraint “auth_permission_pkey”
DETAIL: Key (id)=( ) already exists