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Asked: 2026-05-30T02:40:25+00:00

Using Django with a PostgreSQL (8.x) backend, I have a model where I need

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Using Django with a PostgreSQL (8.x) backend, I have a model where I need to skip a block of ids, e.g. after giving out 49999 I want the next id to be 70000 not 50000 (because that block is reserved for another source where the instances are added explicitly with id – I know that’s not a great design but it’s what I have to work with).

What is the correct/safest place for doing this?

I know I can set the sequence with

SELECT SETVAL(
    (SELECT pg_get_serial_sequence('myapp_mymodel', 'id')),
    70000,
    false
);

but when does Django actually pull a number from the sequence?
Do I override MyModel.save(), call its super and then grab me a cursor and check with

SELECT currval(
    (SELECT pg_get_serial_sequence('myapp_mymodel', 'id'))
);

?

I believe that a sequence may be advanced by django even if saving the model fails, so I want to make sure whenever it hits that number it advances – is there a better place than save()?

P.S.: Even if that was the way to go – can I actually figure out the currval for save()’s session like this? if I grab me a connection and cursor, and execute that second SQL statement, wouldn’t I be in another session and therefore not get a currval?

Thank you for any pointers.

EDIT: I have a feeling that this must be done at database level (concurrency issues) and posted a corresponding PostgreSQL question – How can I forward a primary key sequence in PostgreSQL safely?

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1 Answer

  1. Editorial Team

    My next thought was to not use a sequence for the primary key, but rather always specify the id explicitly from a separate counter table which I check/update before using its latest number – that should be safe from concurrency issues. The only problem is that although I have a single place where I add model instances, other parts of django or third-party apps may still rely on an implicit id, which I don’t want to break.

    But that same mechanism happens to be easily implemented on postgres level – I believe this is the solution:

    • Don’t use SERIAL for the primary key, use DEFAULT my_next_id()
    • Follow the same logic as for “single level gapless sequence” – http://www.varlena.com/GeneralBits/130.php – my_next_id() does an update followed by a select
    • Instead of just increasing by 1, check if a boundary was crossed and if so, increase even further
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