How to define a Django model field with a PostgreSQL function as default value

Question

Is it possible to write a Django model with a field that uses a PostgreSQL function as its default value? Here's a simple example using txid_current() as a default value:

% psql
mydb=# CREATE TABLE test (
    label text default '', 
    txid BIGINT DEFAULT txid_current()
);
CREATE TABLE
mydb=# \d test
           Table "public.test"
 Column |  Type  |       Modifiers        
--------+--------+------------------------
 label  | text   | default ''::text
 txid   | bigint | default txid_current()

mydb=# INSERT INTO test (label) VALUES ('mylabel');
INSERT 0 1
mydb=# select * from test;
  label  |  txid  
---------+--------
 mylabel | 192050
(1 row)

A Django model for that table might look like

class Test(models.Model):
    label = TextField('Label', default='')
    txid = BigIntegerField('txid', default=???)

Is there a way to specify the database function as a default value or do I need to add the default in PostgreSQL as a separate step after running syncdb?

Danny W. Adair · Accepted Answer · 2012-03-13 05:09:30Z

4

You can specify a callable as "default" https://docs.djangoproject.com/en/dev/ref/models/fields/#default

and let your callable execute raw SQL to get the value you're after https://docs.djangoproject.com/en/dev/topics/db/sql/#executing-custom-sql-directly

Regarding your txid_current(), make sure to read https://docs.djangoproject.com/en/dev/topics/db/transactions/#django-s-default-transaction-behavior

answered Mar 13, 2012 at 5:09

Danny W. Adair

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1 Comment

DataGreed Over a year ago

How do I get the database that I should use to execute the function?

Chris Pratt · Accepted Answer · 2011-11-11 16:37:46Z

1

You have to do it in SQL yourself. Django doesn't support that. It's a feature of it's database independence.

answered Nov 11, 2011 at 16:37

Chris Pratt

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1 Comment

David Over a year ago

Thanks. I've only ever used PostgreSQL with Django but knew that MySQL & sqlite both have functions. I see now that MySQL doesn't allow functions as default values so my example really would be PostgreSQL specific. Being able to pass some additional details to insert into the CREATE TABLE statement, depending on what your back end understands, seemed plausible.

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