Django + South - migration can't cast the column type - wants me to use "USING"

Question

I'm getting this error when I try to run ./manage.py migrate my_app:

django.db.utils.ProgrammingError: column "associated" cannot be cast automatically to type integer
HINT: Specify a USING expression to perform the conversion.

That sounds like great advice, but I don't know exactly what South wants me to do. Am I supposed to manually edit the auto-generated python migration file? If so, how -- I don't see standard SQL in that file. Instead, I see this:

# Changing field 'MyTable.associated'
db.alter_column(u'data_mytable', 'associated', 
    self.gf('django.db.models.fields.IntegerField')()
)

Also, why didn't South pick up on this and add the USING clause automatically?

Answer 1

This was my dirty hack:

1. Manually added a migraton for removing the offendig field i.e

migrations.RemoveField( model_name='name_of_the_model', name='offending_field', ) 2. Then I changed the operation of the complaining migration from AlterField to AddField i.e

migrations.AddField(
        model_name='name_of_model',
        name='name_of_field',
        field=models.ForeignKey(blank=True, null=True, to='ForeignKeyModel'),
        preserve_default=False,
    ),

```

Answer 2

You have to substitute original db.alter_column with a db.execute:

# Changing field 'MyTable.associated'
# db.alter_column(u'data_mytable', 'associated', 
#     self.gf('django.db.models.fields.IntegerField')()
# )
db.execute(
    'ALTER TABLE "data_mytable" '
    'ALTER COLUMN "associated" DROP DEFAULT, '
    'ALTER COLUMN "associated" DROP NOT NULL, '
    'ALTER COLUMN "associated" TYPE INTEGER '
    'USING '
    'CASE '
    '  WHEN FALSE THEN 0 ELSE 1 '
    'END'
) 

Of course, WHEN condition may change depending of the associated original type.
See also this link.