Django change database field from integer to CharField

Question

I have a Django app with a populated (Postgres) database that has an integer field that I need to change to a CharField. I need to start storing data with leading zeros in this field. If I run migrate (Django 1.8.4), I get the following error:

psycopg2.ProgrammingError: operator does not exist: character varying >= integer

HINT: No operator matches the given name and argument type(s). You might need to add explicit type casts.

I tried searching Google, but didn't really find much help. I don't really know what I'm supposed to do here. Can anyone help?

Answer 1

You'll need to generate a schema migration. How you do that will depend on which version of Django you are using (versions 1.7 and newer have built-in migrations; older versions of Django will use south ).

Of Note : if this data is production data, you'll want to be very careful with how you proceed. Make sure you have a known good copy of your data you can reinstall if things get hairy. Test your migrations in a non-production environment. Be. Careful!

As for the transformation on the field (from IntegerField to CharField) and the transformation on the field values (to be prepended with leading zeroes) - Django cannot do this for you, you'll have to write this manually. The proper way to do this is to use the django.db.migrations.RunPython migration command ( see here ).

My advice would be to generate multiple migrations; one that creates a new IntegerField, my_new_column and then write to this new column via RunPython. Then, run a second migration that removes the original CharField my_old_column and renames my_new_column as my_old_column .

Answer 2

Originally, I thought that there would be a simple solution where Django or Postgres would do the conversion automatically, but it appears that it doesn't work that way. I think some of suggestions made by others might have worked, but I came up with a simple solution of my own. This was done on a production database so I had to be careful. I ended up adding the character field to the model and did the migration. I then ran a small script in the python shell that copied the data in the integer field, did the conversion that I needed, then wrote that to the new field in the same record in the production database.

for example:

members = Members.objects.all()
for mem in members:
    mem.memNumStr = str(mem.memNum)
    ... more data processing here
    mem.save()

So now, I had the data duplicated in the table in a str field and an int field. I could then modify the views that accessed that field and test it on the production database without breaking the old code. Once that is done, I can drop the old int field. A little bit involved, but pretty simple in the end.

Answer 3

from django 2.x you just need to change the field type from

 IntegerField to CharField

and django will automatically alter field and migrate data as well for you.

Answer 4

I thought a full code example would be helpful. I followed the approach outlined by ken-koster in the comment above. I ended up with two migrations (0091 and 0092). It seems that the two migrations could be squashed into one migration but I did not go that far. (Maybe Django does this automatically but the framework here could be used in case the string values are more complicated than a simple int to string conversion. Also I included a reverse conversion example.)

First migration (0091):

from django.db import migrations, models

class Migration(migrations.Migration):

    dependencies = [
        ('myapp', '0090_auto_20200622_1452'),
    ]

    operations = [
        # store original values in tmp fields
        migrations.RenameField(model_name='member',
                               old_name='mem_num',
                               new_name='mem_num_tmp'),
        # add back fields as string fields
        migrations.AddField(
            model_name='member',
            name='mem_num',
            field=models.CharField(default='0', max_length=64, verbose_name='Number of members'),
        ),
    ]

Second migration (0092):

from django.db import migrations

def copyvals(apps, schema_editor):
    Member = apps.get_model("myapp", "Member")
    members = Member.objects.all()
    for member in members:
        member.rotate_xy = str(member.mem_num_tmp)
        member.save()

def copyreverse(apps, schema_editor):
    Member = apps.get_model("myapp", "Member")
    members = Member.objects.all()
    for member in members:
        try:
            member.mem_num_tmp = int(member.mem_num)
            member.save()
        except Exception:
            print("Reverse migration for member %s failed." % member.name)
            print(member.mem_num)

class Migration(migrations.Migration):

    dependencies = [
        ('myapp', '0091_custom_migration'),
    ]

    operations = [
        # convert integers to strings
        migrations.RunPython(copyvals, reverse_code=copyreverse),

        # remove the tmp field
        migrations.RemoveField(
            model_name='member',
            name='mem_num_tmp',
        ),
    ]