Issue

I have the following existing twitter field on the extended UserProfile model and I’d like to change the field type from a URLField to a CharField with max_length of 20. When I try to migrate the change, I get the error django.db.utils.DataError: value too long for type character varying(20). I do not care about existing data in that field and prefer they go blank if there is existing data when migrating. How can I change the field type and clear existing data?

class UserProfile(models.Model):
    user = models.OneToOneField(User, on_delete=models.SET_NULL, null=True)

    # old field
    twitter = models.URLField(verbose_name="Twitter", blank=True)

    # new field
    # twitter = models.CharField(
    #     max_length=20, verbose_name="Twitter Username", null=True, blank=True
    # )

Solution

Approach 1st

In order to change to type and maximum length of a field which should know the longest url length in your db.
If you are using sqlite3 database then run the following query in your db_shell or sqlite3 shell or whatever you have.

SELECT MAX(LENGTH(column_name)) FROM table_name;

and then set the max_length of the new field to that value.

Approach 2nd

python manage.py shell

and then

# Import your model
profiles = UserProfile.objects.all()
max_length = 0
for userprofile in userprofiles:
    if len(userprofile.twitter) > max_length:
        max_length = len(userprofile.twitter)
print(max_length)

and then set your max_length to the value printed from the above code.

Answered By – shah sawood

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