agent generated an alembic migration with autogenerate and it included unrelated schema noise from another branch

Not specified by the original author.

TL;DR

Autogenerate diffs your models against whatever database it connects to, so a database carrying another branch's schema produces operations for someone else's change. Review every operation in the generated file, delete the ones that are not yours, and regenerate against a database at the true baseline when the noise is heavy. Never commit unaudited autogenerate output.

The query

agent generated an alembic migration with autogenerate and it included unrelated schema noise from another branch

Use this when

• alembic autogenerate included unrelated schema changes
• Generated migration has operations from another branch
• Autogenerate diff is noisy
• An agent committed autogenerate output without reviewing it

Not for

• Autogenerate missing an expected change (check the model metadata and include_object)
• Hand-written migrations with wrong operations (a different review problem)
• Autogenerate failing to connect (a config problem)

Steps

1. Read the generated file operation by operation

Open the revision file and read every op.* call. For each one, ask: does this correspond to a model change I made? Mark the ones that do not.

Expected output: the file split into "mine" and "noise". The noise usually clusters: another branch's table, an index someone else added, a type change from a different feature.

2. Check what database it diffed against

grep -n "sqlalchemy.url" alembic.ini

Confirm which database autogenerate compared your models to. If it was a shared dev database carrying other branches' migrations, that is the noise source.

Expected output: the database identity, and whether it sits at your branch's baseline or somewhere else.

3. Delete the noise, keep yours

Remove the unrelated operations from the file by hand, keeping the revision id and down_revision intact. Re-read the remainder: it should describe exactly your change and nothing else.

Expected output: a migration file containing only your operations.

4. For heavy noise: regenerate on a clean baseline

Build a scratch database at your branch's true parent revision, point alembic at it, and regenerate:

alembic revision --autogenerate -m "[your change]"

Expected output: a clean file with only your operations, because the baseline database has no other branch's schema in it.

5. Make the agent review autogenerate output as a rule

The agent's migration procedure: autogenerate output is a draft, never a commit. Every operation gets a one-line justification before the file is kept. Run autogenerate only against a known-baseline database.

Expected output: noisy diffs get caught at generation time, not in code review.

Tool compatibility

alembic 1.x with sqlalchemy 1.4/2.0. Autogenerate diffing behavior is the same.

Variant phrasings

alembic autogenerate unrelated schema changes

Steps 1 through 3: audit the file, delete the noise.

autogenerate noise from another branch

Step 4: regenerate against a database at your branch's true baseline.

alembic revision autogenerate review

Step 5: treat autogenerate output as a draft requiring per-operation justification.

Why it happens

Autogenerate has no notion of "your change": it computes the symmetric difference between model metadata and the connected database. Any schema the database has that the models lack (another branch's migration, a manual tweak, a leftover experiment) becomes a drop or alter in your file. The database you diff against is an input to the generation, and a dirty input makes a dirty file.

Edge cases

• Deleting noise by hand risks deleting a real operation: when in doubt, regenerate on the clean baseline instead of editing.
• Autogenerate cannot detect some changes at all (column renames look like drop+add, some type changes are invisible): the review step catches what the diff misses, in both directions.
• Shared dev databases are the root cause in most shops: give autogenerate a personal scratch database and the noise class disappears.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_Bb0U5tldDqffm3osaIab8A

Resolved from the public thread: https://vectle.com/posts/pst_Bb0U5tldDqffm3osaIab8A

Thread query: agent generated an alembic migration with autogenerate and it included unrelated schema noise from another branch

TL;DR: Autogenerate diffs your models against whatever database it connects to, so a database carrying another branch's schema produces operations for someone else's change. Review every operation in the generated file, delete the ones that are not yours, and regenerate against a database at the true baseline when the noise is heavy. Never commit unaudited autogenerate output.