Can not longer "downgrade base" in 1.6.0

[nhumrich]

Describe the bug
alembic downgrade no longer recognizes the revision "base", and likewise, you can no longer downgrade completely.
When you run downgrade base, you get an error that the revision base is not found.

This works great on any previous alembic version. Is broken in 1.6.0

Expected behavior
alembic downgrade base should downgrade all revisions.

To Reproduce
Install the alembic 1.6.0, have some revisions, upgrade to head, then run alembic downgrade base.

# Copy error here. Please include the full stack trace.
INFO  [alembic.runtime.migration] Context impl PostgresqlImpl.
INFO  [alembic.runtime.migration] Will assume transactional DDL.
ERROR [alembic.util.messaging] Destination base is not a valid downgrade target from current head(s)
  FAILED: Destination base is not a valid downgrade target from current head(s)

Versions.

• OS: linux
• Python: 3.9.4
• Alembic: 1.6.0
• SQLAlchemy: 1.4.13
• Database: postgres
• DBAPI: psycopg2

[zzzeek]

hi there -

We need to see a revision structure that reproduces this. "alembic downgrade base" is of course tested and working with a fresh set of files as below, there's no issue:

[classic@photon3 alembic:master]$ PYTHONPATH=~/dev/sqlalchemy/lib/ python -m alembic init foo
  Creating directory /home/classic/dev/alembic/foo ...  done
  Creating directory /home/classic/dev/alembic/foo/versions ...  done
  Generating /home/classic/dev/alembic/foo/script.py.mako ...  done
  Generating /home/classic/dev/alembic/foo/README ...  done
  File /home/classic/dev/alembic/alembic.ini already exists, skipping
  Generating /home/classic/dev/alembic/foo/env.py ...  done
  Please edit configuration/connection/logging settings in '/home/classic/dev/alembic/alembic.ini' before proceeding.
[classic@photon3 alembic:master]$ PYTHONPATH=~/dev/sqlalchemy/lib/ python -m alembic revision -m "rev1" 
INFO  [alembic.runtime.migration] Context impl SQLiteImpl.
INFO  [alembic.runtime.migration] Will assume non-transactional DDL.
INFO  [sqlalchemy.engine.Engine] PRAGMA main.table_info("alembic_version")
INFO  [sqlalchemy.engine.Engine] [raw sql] ()
INFO  [sqlalchemy.engine.Engine] SELECT alembic_version.version_num 
FROM alembic_version
INFO  [sqlalchemy.engine.Engine] [generated in 0.00009s] ()
INFO  [sqlalchemy.engine.Engine] BEGIN (implicit)
INFO  [sqlalchemy.engine.Engine] PRAGMA main.table_info("alembic_version")
INFO  [sqlalchemy.engine.Engine] [raw sql] ()
INFO  [sqlalchemy.engine.Engine] COMMIT
  Generating /home/classic/dev/alembic/foo/versions/ee80fc2540c6_rev1.py ...  done
  Running post write hook "black" ...
reformatted /home/classic/dev/alembic/foo/versions/ee80fc2540c6_rev1.py
All done! ✨ 🍰 ✨
1 file reformatted.
  done
  Running post write hook "zimports" ...
[Writing]       /home/classic/dev/alembic/foo/versions/ee80fc2540c6_rev1.py ([18% of lines are imports] [source +0L/-1L] [1 imports removed in 0.0023 sec])
  done
[classic@photon3 alembic:master]$ PYTHONPATH=~/dev/sqlalchemy/lib/ python -m alembic revision -m "rev2" 
INFO  [alembic.runtime.migration] Context impl SQLiteImpl.
INFO  [alembic.runtime.migration] Will assume non-transactional DDL.
INFO  [sqlalchemy.engine.Engine] PRAGMA main.table_info("alembic_version")
INFO  [sqlalchemy.engine.Engine] [raw sql] ()
INFO  [sqlalchemy.engine.Engine] SELECT alembic_version.version_num 
FROM alembic_version
INFO  [sqlalchemy.engine.Engine] [generated in 0.00008s] ()
INFO  [sqlalchemy.engine.Engine] BEGIN (implicit)
INFO  [sqlalchemy.engine.Engine] PRAGMA main.table_info("alembic_version")
INFO  [sqlalchemy.engine.Engine] [raw sql] ()
INFO  [sqlalchemy.engine.Engine] COMMIT
  Generating /home/classic/dev/alembic/foo/versions/1e3337826fb7_rev2.py ...  done
  Running post write hook "black" ...
reformatted /home/classic/dev/alembic/foo/versions/1e3337826fb7_rev2.py
All done! ✨ 🍰 ✨
1 file reformatted.
  done
  Running post write hook "zimports" ...
[Writing]       /home/classic/dev/alembic/foo/versions/1e3337826fb7_rev2.py ([18% of lines are imports] [source +0L/-1L] [1 imports removed in 0.0022 sec])
  done
[classic@photon3 alembic:master]$ PYTHONPATH=~/dev/sqlalchemy/lib/ python -m alembic upgrade head
INFO  [alembic.runtime.migration] Context impl SQLiteImpl.
INFO  [alembic.runtime.migration] Will assume non-transactional DDL.
INFO  [sqlalchemy.engine.Engine] PRAGMA main.table_info("alembic_version")
INFO  [sqlalchemy.engine.Engine] [raw sql] ()
INFO  [sqlalchemy.engine.Engine] SELECT alembic_version.version_num 
FROM alembic_version
INFO  [sqlalchemy.engine.Engine] [generated in 0.00007s] ()
INFO  [sqlalchemy.engine.Engine] BEGIN (implicit)
INFO  [sqlalchemy.engine.Engine] PRAGMA main.table_info("alembic_version")
INFO  [sqlalchemy.engine.Engine] [raw sql] ()
INFO  [sqlalchemy.engine.Engine] COMMIT
INFO  [sqlalchemy.engine.Engine] BEGIN (implicit)
INFO  [alembic.runtime.migration] Running upgrade  -> ee80fc2540c6, rev1
INFO  [sqlalchemy.engine.Engine] INSERT INTO alembic_version (version_num) VALUES ('ee80fc2540c6')
INFO  [sqlalchemy.engine.Engine] [generated in 0.00008s] ()
INFO  [sqlalchemy.engine.Engine] COMMIT
INFO  [sqlalchemy.engine.Engine] BEGIN (implicit)
INFO  [alembic.runtime.migration] Running upgrade ee80fc2540c6 -> 1e3337826fb7, rev2
INFO  [sqlalchemy.engine.Engine] UPDATE alembic_version SET version_num='1e3337826fb7' WHERE alembic_version.version_num = 'ee80fc2540c6'
INFO  [sqlalchemy.engine.Engine] [generated in 0.00007s] ()
INFO  [sqlalchemy.engine.Engine] COMMIT
[classic@photon3 alembic:master]$ PYTHONPATH=~/dev/sqlalchemy/lib/ python -m alembic downgrade base
INFO  [alembic.runtime.migration] Context impl SQLiteImpl.
INFO  [alembic.runtime.migration] Will assume non-transactional DDL.
INFO  [sqlalchemy.engine.Engine] PRAGMA main.table_info("alembic_version")
INFO  [sqlalchemy.engine.Engine] [raw sql] ()
INFO  [sqlalchemy.engine.Engine] SELECT alembic_version.version_num 
FROM alembic_version
INFO  [sqlalchemy.engine.Engine] [generated in 0.00008s] ()
INFO  [sqlalchemy.engine.Engine] BEGIN (implicit)
INFO  [alembic.runtime.migration] Running downgrade 1e3337826fb7 -> ee80fc2540c6, rev2
INFO  [sqlalchemy.engine.Engine] UPDATE alembic_version SET version_num='ee80fc2540c6' WHERE alembic_version.version_num = '1e3337826fb7'
INFO  [sqlalchemy.engine.Engine] [generated in 0.00008s] ()
INFO  [sqlalchemy.engine.Engine] COMMIT
INFO  [sqlalchemy.engine.Engine] BEGIN (implicit)
INFO  [alembic.runtime.migration] Running downgrade ee80fc2540c6 -> , rev1
INFO  [sqlalchemy.engine.Engine] DELETE FROM alembic_version WHERE alembic_version.version_num = 'ee80fc2540c6'
INFO  [sqlalchemy.engine.Engine] [generated in 0.00007s] ()
INFO  [sqlalchemy.engine.Engine] COMMIT

[zzzeek]

cc @simonbowly

[nhumrich]

Here is my revision chain, its just a linear graph, no branches.

2495d6ed2fca -> 93db6fd0a57f (head), add description
c58c10dde263 -> 2495d6ed2fca, increse limit
3094b6d27bc4 -> c58c10dde263, components
2774eb193274 -> 3094b6d27bc4, datamigration2
f8d63b07d4d1 -> 2774eb193274, datamigration1
e7aa88f1f76f -> f8d63b07d4d1, id on components
434c3ca80478 -> e7aa88f1f76f, Remove role
a8312bc2775a -> 434c3ca80478, commenting system
5b16eb9c9524 -> a8312bc2775a, config is specific
c8213684c654 -> 5b16eb9c9524, conversion migration
7b34feaad1eb -> c8213684c654, add default column
54cc14b31868 -> 7b34feaad1eb, id should be text
90785ecd62d7 -> 54cc14b31868, add components table
38a452b30df8 -> 90785ecd62d7, add records table
b204470cfb8f -> 38a452b30df8, natural sort things
<base> -> b204470cfb8f, initial structure```

[zzzeek]

is your project open source ?

[nhumrich]

no. Let me see if I can reproduce somehow

[nhumrich]

Ok, I found the issue. The issue is when the database is completely clean, aka, there is no alembic_version table in the database.

We run a "reset database" script that cleans the database in a test environment. Sometimes the database is already clean. Before, downgrade base would work, even on a clean database. Now, it breaks if there isn't a alembic_versions table.

[zzzeek]

OK that sounds more localized, let's see

[zzzeek]

OK! sure I just run "downgrade base" again and I get the error :)

[sqla-tester]

Mike Bayer has proposed a fix for this issue in the master branch:

narrow rangenotancestor to exclude target heads already present https://gerrit.sqlalchemy.org/c/sqlalchemy/alembic/+/2810