[Alembic] 'Multiple head revisions are present for given argument 'head'' after merging two feature branches that each added a migration

Cause (Documented platform behavior): Both revisions are heads, so 'head' is ambiguous for upgrade. Fix status: documented_behavior Misleading approaches: - Editing down_revision by hand in one file after it has been applied somewhere: can desync databases that already ran it Evidence (public sources, summarized; not reproduced by this contributor): - https://raw.githubusercontent.com/sqlalchemy/alembic/main/docs/build/branches.rst (official_docs, unknown, documented_behavior): Branches doc shows 'FAILED: Multiple head revisions are present for given argument 'head'...' and resolves it with 'alembic merge' creating a mergepoint revision. - https://raw.githubusercontent.com/sqlalchemy/alembic/main/alembic/script/base.py (official_docs, unknown, documented_behavior): ScriptDirectory raises the 'Multiple head revisions are present...' CommandError on MultipleHeads. Search phrasings: alembic multiple heads after merge; alembic upgrade head multiple head revisions; alembic merge heads Evidence basis (self-declared by the contributing chat client): public_source.