Django + PostgreSQL: How to reset primary key? - django

Question

I have been working on an application in Django. To begin with, for simplicity, I had been using sqlite3 for the database.
However, once I moved to PostgreSQL, I've run into a bit of a problem: the primary key does not reset once I clear out a table.
This app is a game that is played over a long time period (weeks). As such, every time a new game starts, all of the data is cleared out of the database and then new, randomized data is added.
I'd like to be able to "start over" with primary keys starting at 1 each time I clean/rebuild the game.
The code still works as-is, but integers are a pretty natural way for describing the objects in my game. I'd like to have each new game start at 1 rather than wherever the last game left off.
How can I reset the primary key counter in PostgreSQL? Keep in mind that I don't need to preserve the data in the table since I am wiping it out anyway.

Answer 1

In your app directory try this:
python manage.py help sqlsequencereset
Pipe it into psql like this to actually run the reset:
python manage.py sqlsequencereset myapp1 myapp2 | psql
Edit: here's an example of the output from this command on one of my tables:
BEGIN;
SELECT setval('"project_row_id_seq"', coalesce(max("id"), 1), max("id") IS NOT null) FROM "project_row";
COMMIT;

Answer 2

As suggested by "Van Gale" you can get the commands to solve your problem running sqlsequencereset.
or
You can execute the SQL query generated by sqlsequencereset from within python in this way (using the default database):
from django.core.management.color import no_style
from django.db import connection
from myapps.models import MyModel1, MyModel2
sequence_sql = connection.ops.sequence_reset_sql(no_style(), [MyModel1, MyModel2])
with connection.cursor() as cursor:
for sql in sequence_sql:
cursor.execute(sql)
I tested this code with Python3.6, Django 2.0 and PostgreSQL 10.

Answer 3

If you perform a raw sql, can do this:
ALTER SEQUENCE youApp_id_seq RESTART WITH 1;
docs:
http://www.postgresql.org/docs/8.2/static/sql-altersequence.html

Answer 4

I view auto-increment primary keys as purely internal identifiers for database records, and I don't like exposing them to users. Granted, it's a common design to use them as part of URLs, but even there slugs or other identifiers feel more appropriate.

Answer 5

If you do not want to have to manually grab the apps you need, or if you have a series of different databases, this command will dynamically gather all connections from settings.py and reset the sequence.
To run use: python manage.py reset_sequences
import psycopg2
from django.conf import settings
from django.core.management.base import BaseCommand
from django.db import connections
def dictfetchall(cursor):
"""Return all rows from a cursor as a dict"""
columns = [col[0] for col in cursor.description]
return [
dict(zip(columns, row))
for row in cursor.fetchall()
]
class Command(BaseCommand):
help = "Resets sequencing errors in Postgres which normally occur due to importing/restoring a DB"
def handle(self, *args, **options):
# loop over all databases in system to figure out the tables that need to be reset
for name_to_use_for_connection, connection_settings in settings.DATABASES.items():
db_name = connection_settings['NAME']
host = connection_settings['HOST']
user = connection_settings['USER']
port = connection_settings['PORT']
password = connection_settings['PASSWORD']
# connect to this specific DB
conn_str = f"host={host} port={port} user={user} password={password}"
conn = psycopg2.connect(conn_str)
conn.autocommit = True
select_all_table_statement = f"""SELECT *
FROM information_schema.tables
WHERE table_schema = 'public'
ORDER BY table_name;
"""
# just a visual representation of where we are
print('-' * 20, db_name)
try:
not_reset_tables = list()
# use the specific name for the DB
with connections[name_to_use_for_connection].cursor() as cursor:
# using the current db as the cursor connection
cursor.execute(select_all_table_statement)
rows = dictfetchall(cursor)
# will loop over table names in the connected DB
for row in rows:
find_pk_statement = f"""
SELECT k.COLUMN_NAME
FROM information_schema.table_constraints t
LEFT JOIN information_schema.key_column_usage k
USING(constraint_name,table_schema,table_name)
WHERE t.constraint_type='PRIMARY KEY'
AND t.table_name='{row['table_name']}';
"""
cursor.execute(find_pk_statement)
pk_column_names = dictfetchall(cursor)
for pk_dict in pk_column_names:
column_name = pk_dict['column_name']
# time to build the reset sequence command for each table
# taken from django: https://docs.djangoproject.com/en/3.0/ref/django-admin/#sqlsequencereset
# example: SELECT setval(pg_get_serial_sequence('"[TABLE]"','id'), coalesce(max("id"), 1), max("id") IS NOT null) FROM "[TABLE]";
try:
reset_statement = f"""SELECT setval(pg_get_serial_sequence('"{row['table_name']}"','{column_name}'),
coalesce(max("{column_name}"), 1), max("{column_name}") IS NOT null) FROM "{row['table_name']}" """
cursor.execute(reset_statement)
return_values = dictfetchall(cursor)
# will be 1 row
for value in return_values:
print(f"Sequence reset to {value['setval']} for {row['table_name']}")
except Exception as ex:
# will only fail if PK is not an integer...
# currently in my system this is from django.contrib.sessions
not_reset_tables.append(f"{row['table_name']} not reset")
except psycopg2.Error as ex:
raise SystemExit(f'Error: {ex}')
conn.close()
print('-' * 5, ' ALL ERRORS ', '-' * 5)
for item_statement in not_reset_tables:
# shows which tables produced errors, so far I have only
# seen this with PK's that are not integers because of the MAX() method
print(item_statement)
# just a visual representation of where we are
print('-' * 20, db_name)

Answer 6

You need to truncate the table.
See http://www.postgresql.org/docs/8.1/static/sql-truncate.html