Ecto migration rollbacks: Is automatic safe rollback for schema changes supported?

Answer

Ecto does **not** provide an automatic, safe rollback for most schema changes. Only a narrow set of reversible edits can be auto‑generated by the change/0 callback; everything else requires an explicit down/1 implementation to avoid data loss.

Confirmed facts

• The change/0 callback auto‑generates reverse operations for: add/remove column, create/drop table, add/drop index, create/drop constraint.
• Column renames (rename/3), column type changes, adding a NOT NULL constraint without a default, and dropping a column that contains data **do not** have auto‑generated safe rollbacks.
• The :force option in alter table or type‑change clauses bypasses type checking and can silently truncate or discard data; it is not safe for production unless intentional data loss is acceptable.

Likely explanation

Ecto’s design treats migrations as explicit scripts. The change/0 helper can infer inverses only for operations that are information‑preserving (e.g., adding a column). Operations that may lose or alter data cannot be safely inverted without knowing how to convert existing values, so Ecto leaves the responsibility to the developer.

Steps to verify the current behavior (optional)

1. Create a migration that adds a column via change/0, run mix ecto.migrate, insert sample data, then run mix ecto.rollback and confirm the column is removed and data remains.
2. Create a migration that renames a column using rename/3 in change/0, migrate, insert data, rollback, and observe that the column name does not revert (manual down required).
3. Create a migration that changes a column type with :force, insert data exceeding the new type’s length, migrate, rollback, and check whether data is truncated; compare with a manual down that safely casts values.

If you are using change/0 exclusively, you only need to worry about the constructs listed above; otherwise, ensure each migration includes a matching down/1 that handles data conversion or preservation.