Tip: Renaming Model Fields in Django

Did you ever have to re-purpose a column in your database schema? Here's a quick and easy way to do this if you happen to be using Django.

Scenario

I had an integer field in my model called lines which counted the lines of code in a particular tar.gz package. I figured the file size is a better indicator so decided to start using it. I was not planning to use the old field anymore and I didn't care about the data it was holding. So I decided to re-purpose it as the size field.

Possible methods

Looking around I figured several different ways to do this:

1. Continue using the existing lines field and keep referencing the old name in the code. This is no-brainer but feels awkward and is a disaster waiting to happen;
2. Add new size field and remove the old lines field. This involves modification to the DB schema and requires at least a backup with possible down time. Not something I will jump at right away;
3. Add a size property in the model class which will persist to self.lines. This is a quick way to go but I'm not sure if one can use the property with the Django QuerySet API (objects.filter(), objects.update(), etc.) I suspect not. If you don't filter by the property or use it in bulk operations this method is fine though;
4. Change the field name to size but continue to use the lines DB column; Mind my wording here :);
5. Rename the column in the DB schema and then update the model class field.

How I did it

I decided to go for option 4 above: change the field name to size but continue to use the lines DB column.

diff --git a/models.py b/models.py
index e06d2b2..18cad6f 100644
--- a/models.py
+++ b/models.py
@@ -667,7 +667,7 @@ class Package(models.Model):
-    lines = models.IntegerField(default=None, null=True, blank=True)
+    size  = models.IntegerField(default=None, null=True, blank=True, db_column='lines')

1. Removed all references to lines from the code except the model class. This served as clean up as well.
2. Renamed the model field to size but continued using the lines DB column as shown above. Django's db_column option makes this possible.
3. From the Django shell (./manage.py shell) reset size to None (NULL) for all objects;
4. Finally implement my new code and functionality behind the size field.

The entire process happened for under 10 minutes. I will also opt for renaming the DB column at a later time. This is to sync the naming used in Python code and in MySQL in case I ever need to use raw SQL or anything but Django.

If you were me, how would you do this? Please share in the comments below.