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circular : A circular data dependency

An example of a restore.py that abandoned before 20250731 because the database contains a circular dependency. Since 20250731 Lino handles this situation by saving “intermediate” model instances when appropriate.

Here is our models.py file:

from lino.api import dd


class A(dd.Model):
    b = dd.ForeignKey('circular.B', related_name="a2b", blank=True, null=True)


class B(dd.Model):
    a = dd.ForeignKey('circular.A', related_name="b2a", blank=True, null=True)

And a demo fixture:

from lino.api import rt


def objects():
    A = rt.models.circular.A
    B = rt.models.circular.B
    yield (a := A())
    yield (b := B(a=a))
    a.b = b
    yield a

These two database rows exist, and Lino can dump them, but the dump wasn’t able to load before 20250731.

>>> from lino import startup
>>> startup('lino_book.projects.circular.settings')
>>> from lino.api.doctest import *
>>> from atelier.sheller import Sheller
>>> dbhash.check_virgin()
>>> shell = Sheller(settings.SITE.project_dir)
>>> assert (shell.cwd / 'manage.py').exists()

>>> shell("django-admin ddt -b")
...
Writing /.../circular/tmp/a/restore.py...
...
There are 2 models with circular dependencies :
- circular.A (1 rows, depends on circular.B)
- circular.B (1 rows, depends on circular.A)


Wrote 2 objects to /.../circular/tmp/b/restore.py and siblings.
Successfully ran double-dump test in /.../circular/tmp.

Restore database state (dbhash doesn’t work after above snippet because the old database is still loaded):

>>> shell("django-admin prep -b")
...
`initdb demo` started on database /.../circular/default.db.
...
Installed 3 object(s) from 1 fixture(s)

Note that it reports 3 (not 2) objects. That’s because the A instance is being yielded twice.