Setting database level defaults for fields in django?

Question

What methods do you use to set up database level defaults in django?

I understand this is a django wontfix decision, but there must be some ideas floating around about a way to implement this if written for a specific backend (postgres).

I'd like to power this through python so I'd prefer not to use /sql/.sql files.

I was thinking I might set attributes on existing fields such as:

foo = models.CharField(max_length=256, blank=True)
foo.db_default = ''

Then filter through fields and construct an ALTER COLUMN SET DEFAULT query.

Where would I put that though? The docs say not to alter the database in the post_syncdb hook.

Any ideas where to put this action? I'm also thinking I could generate the sql/.sql files dynamically. A management command would be better than nothing. Automatic would be great. Hmm I wonder if post_syncdb can generate sql files in time to be executed? Will test.

Yuji 'Tomita' Tomita · Accepted Answer · 2012-06-08 03:40:39Z

I tried doing post_syncdb but it was too late to use .sql files. Hooking into class_prepared worked out in time for syncdb but I realized I was overcomplicating things. I might as well run some code once at model instantiation.

I ended up with a straight forward solution that can be easily understood in 6 months when I need to fiddle with this again.

def db_default(field, db_default):
    """
    Set attribute __db_default for signal to set default values with.
    """
    field.__db_default = db_default
    return field

class OrderShipLog(models.Model):
    date = db_default(models.DateTimeField(), 'now()')

def generate_default_sql(model):
    """
    Generate SQL for postgresql_psycopg2 default values because the shipping
    database post doesn't guarantee posting certain fields but id like them all to be strings.
    """
    db_fields = filter(lambda x: hasattr(x, '__db_default'), model._meta.fields)
    db_table = model._meta.db_table
    modelname = model._meta.object_name

    sql_dir = os.path.join(os.path.dirname(__file__), 'sql/')
    sql_filename = os.path.join(sql_dir, '{modelname}.postgresql_psycopg2.sql'.format(
        modelname=modelname.lower()))

    from django.db import connection
    if not db_table in connection.introspection.table_names():
        print >> sys.stderr, "{0} not in introspected table list; creating db default sql file : {1}".format(db_table, sql_filename)
        try:
            if not os.path.exists(sql_dir):
                os.makedirs(sql_dir)

            with open(sql_filename, 'w+') as f:
                for field in db_fields:
                    attname, db_column = field.get_attname_column()
                    if not field.__db_default:
                        raise Exception("Must enter value for field {modelname}.{field}.__db_default".format(
                            modelname=modelname,
                            field=attname))
                    f.write('ALTER TABLE {db_table} ALTER COLUMN {db_column} SET DEFAULT {default};\n'.format(
                        db_table=db_table,
                        db_column=db_column,
                        default=field.__db_default,
                        ))
        except Exception, e:
            print >> sys.stderr, "Could not generate SQL file. Manually set the defaults! {0}\n".format(e) * 10

generate_default_sql(OrderShipLog)

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