Downgrade for just one branch from multi heads

[sqlalchemy-bot]

Migrated issue, originally created by dem0n3d (dem0n3d)

I frequently need possibility to downgrade just one of my heads to branchpoint, but alembic tries to downgrade all of them. Can you implement this feature, please?

[sqlalchemy-bot]

Michael Bayer (zzzeek) wrote:

unfortunately this is extremely complicated as there are scenarios where those other branches must be pulled down. I will leave this open but I'm unlikely to have a good solution for this anytime soon.

[sqlalchemy-bot]

Changes by Michael Bayer (zzzeek):

• added labels: versioning model

[sqlalchemy-bot]

Michael Bayer (zzzeek) wrote:

the idea is that given:

#!python

base -> rev1 --> rev2 (brancha)
             +-> rev2a (branchb) -> rev3a

if im on "rev2" / "rev3a" and i say:

alembic downgrade branchb@rev1

it would figure out we don't need to do rev2.

A quick patch that does this is:

#!diff

diff --git a/alembic/script/revision.py b/alembic/script/revision.py
index 3d9a332..7d609c1 100644
--- a/alembic/script/revision.py
+++ b/alembic/script/revision.py
@@ -655,6 +655,10 @@ class RevisionMap(object):
 
         uppers = util.dedupe_tuple(self.get_revisions(upper))
 
+        if select_for_downgrade and "@" in lower and "@" not in upper:
+            branch = lower.split("@")[0]
+            uppers = set(u for u in uppers if branch in u.branch_labels)
+
         if not uppers and not requested_lowers:
             return

but this breaks tests for the situation where we do need to still pull down rev2, including if we said to go all the way to the base, as well as when other branches have dependencies on one of the revisions that we're removing. so it would need to figure out "preferred" heads to bring down but also still figure out other heads that have to come down.

[sqlalchemy-bot]

dem0n3d (dem0n3d) wrote:

I wish to alembic have operation like git "checkout": when you specify revision (or branch), alembic ensures that db migrated to this state (and only this). But I realize that it is too complicated :)

Maybe somethink like "downgrade revision range"?

alembic downgrade rev2a:rev3a

Or simply add some CLI options to your solution?