Django CharField with dynamic default value-CodePudding

I want to add a new field to a PostgreSQL database.

It's a not null and unique CharField, like

dyn = models.CharField(max_length=31, null=False, unique=True)

The database already has relevant records, so it's not an option to

delete the database
reset the migrations
wipe the data
set a default static value.

How to proceed?

Edit

Tried to add a default=uuid.uuid4

dyn = models.CharField(max_length=31, null=False, unique=True, default=uuid.uuid4)

but then I get

Ensure this value has at most 31 characters (it has 36).

Edit 2

If I create a function with .hex (as found here)

def hex_uuid():
    """
    The UUID as a 32-character lowercase hexadecimal string
    """
    return uuid.uuid4().hex

and use it in the default

dyn = models.CharField(max_length=31, null=False, unique=True, default=hex_uuid)

I'll get

Ensure this value has at most 31 characters (it has 32).

Note: I don't want to simply get a substring of the result, like adjusting the hex_uuid() to have return str(uuid.uuid4())[:30], since that'll increase the collision chances.

CodePudding user response：

I ended up using one of the methods shown by Oleg

def dynamic_default_value():
    """
    This function will be called when a default value is needed.
    It'll return a 31 length string with a-z, 0-9.
    """
    alphabet = string.ascii_lowercase   string.digits
    return ''.join(random.choices(alphabet, k=31)) # 31 is the length of the string

with

dyn = models.CharField(max_length=31, null=False, unique=True, default=dynamic_default_value)

If the field was max_length of 32 characters then I'd have used the hex_uuid() present in the question.

If I wanted to make the dynamic field the same as the id of another existing unique field in the same model, then I'd go through the following steps.

CodePudding user response：

you can reset the migrations or edit it or create new one like:

python manage.py makemigrations name_you_want

after that:

python manage.py migrate same_name

edit: example for funcation:

def generate_default_data():
    return datetime.now()

class MyModel(models.Model):
     field = models.DateTimeField(default=generate_default_data)