Field default values are ignored in schema editor (migrations)

[tsifrer]

Describe the bug
If field has a default value specified, schema editor doesn't pass it through to SQL.

To Reproduce
Example model:

class DummyDefault(models.Model):
    default_str = fields.CharField(max_length=255, null=True, default="My default string")
    default_int = fields.IntField(default=1337)
    default_bool = fields.BooleanField(default=True)
    default_datetime_auto_now = fields.DatetimeField(auto_now=True)
    default_datetime_auto_now_add = fields.DatetimeField(auto_now_add=True)

Calling tortoise makemigrations correctly generates the migration file, and also every default is present in the migration file.
However tortoise sqlmigrate and tortoise migrate generate below SQL which is missing all the default values

CREATE TABLE "dummydefault" (
    "id" SERIAL NOT NULL PRIMARY KEY,
    "default_str" VARCHAR(255),
    "default_int" INT NOT NULL,
    "default_bool" BOOL NOT NULL,
    "default_datetime_auto_now" TIMESTAMPTZ NOT NULL,
    "default_datetime_auto_now_add" TIMESTAMPTZ NOT NULL
);

If altering a columns default later on, I've switched default_datetime_auto_now field from auto_now=True to auto_now_add=True it correctly generates a migration file, but the sqlmigrate returns an empty SQL.

Expected behavior
Default values should be present in the SQL.

CREATE TABLE "dummydefault" (
    "id" SERIAL NOT NULL PRIMARY KEY,
    "default_str" VARCHAR(255) DEFAULT 'My default string',
    "default_int" INT NOT NULL DEFAULT 1337,
    "default_bool" BOOL NOT NULL DEFAULT True,
    "default_datetime_auto_now" TIMESTAMPTZ NOT NULL DEFAULT CURRENT_TIMESTAMP,
    "default_datetime_auto_now_add" TIMESTAMPTZ NOT NULL DEFAULT CURRENT_TIMESTAMP
);

This expected sql was generated via from tortoise.utils import get_schema_sql

[abondar]

Hi!

"default" set in python is not propagated to db schema by design - as it limits scope of what "default" is able to do, as right now it can be anything from python, not just something acceptable by database

It works like that in django too, so for us there is principle of least amount of surprises too

But django developed "db_default" fallback for it, probably we should too

[tsifrer]

Fair point. Django really does that yeah. db_default might be a good solution.
The main problem I see right now is that it's not consistent within tortoise itself, as if you use await Tortoise.generate_schemas() will create a different table than if you make it via tortoise migrations.