migrate does not store default values in the database

I'm trying out the migrations in Django 1.7 and I was surprised when my migrations did not set default values in the database. Can someone share the motivation for this functionality? (more details below)

This was surprising given the documentation I read:

1. the release notes for 1.7 suggest using `null=True` for new database columns, but only because of the potential performance impact during a site update (which seems odd, since the migrations don't set default values anyways…)
2. the example of a migration file clearly shows a default value of `0` for the Author.rating field, even though it turns out the default isn't used

Running `python manage.py sqlmigrate appname migrationnumber` for an example like the Author.rating addition in the docs would look something like this:

BEGIN;
ALTER TABLE "author" ADD COLUMN "rating" integer NOT NULL DEFAULT 0;
ALTER TABLE "author" ALTER COLUMN "rating" DROP DEFAULT;

COMMIT;

However, looking in the source code, I can see: "(Django usually does not use in-database defaults)".

I find it quite problematic for production site updates, since my updates generally look like:

1. generate migrations
2. run migrations on prod db, while old webserver code is still running
3. after migrations, update webserver

I cannot afford to have breaking problems between steps 2 & 3 if I want to add a new db column that is not null and has a default value.

Does anyone here know the motivation for the existing no-defaults-in-the-db functionality? I'd like to know, since I'm putting together a backend derived from 'django.db.backends.postgresql_psycopg2' that is exactly the same, except for setting default values in the db, and I'd like to know what concerns there might be for a backend like this?

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