CODEWITHSUNDEEP
pythondjangodjango-models
Ron Moran
Ron Moran

Reputation: 21

Django default model field value

suppose I have a Django object of this type:

class Some(Model):
    some_value = CharField(max_length=20)

and after a while (and a few objects later) I understood I'd like to save the created time for each new object, so I add the following field:

created = FloatField(default=time.time)

Let's suppose now I have a few old object which were created without the "created" field. When I query them, they receive the default time.time, even though it's suppose to be None. How can I get a different value for all the old objects? I only want new objects to be created with this default, and say all old objects without this field to be None, or some other known value. Setting null=True doesn't make a difference.

Thanks!

Upvotes: 2

Views: 3074

Answers (1)

Arman Ordookhani
Arman Ordookhani

Reputation: 6536

If I understand your question correctly, you want to add a created field defaulted to now but want to set it to None for old rows. You could achive this by making two migrations:

  • Add new field and default it to None

      created = FloatField(null=True, default=None)

  • Run makemigrations

  • Change default value for newer fields:

      created = FloatField(null=True, default=time.time)

  • Run makemigrations

  • Run migrate

When first migration run, all old rows created will be set to None (because they don't have a value and default will be used). When seconds migration run old rows will not change because they already has a value. New rows will use the default (time.time).

Without migrations

syncdb does nothing if table is already created in DB. You should modify your DB manually (i.e. ALTER TABLE commands of SQL)

  • Modify your DB and add a nullable created field with NULL as default value.
  • Add field to your code (with default=time.time)

When you add a new nullable column to DB, all existing rows will set to NULL and Django handles default in application level.

Upvotes: 4

Related Questions