db:check-migrations job

This job runs on the test stage of a merge request pipeline. It checks:

  1. A schema dump comparison between the author’s working branch and the target branch, after executing a rollback of your new migrations. This check validates that the schema properly resets to what it was before executing this new migration.
  2. A schema dump comparison between the author’s working branch and the db/structure.sql file that the author committed. This check validates that it contains all the expected changes in the migration.
  3. A Git diff between the db/schema_migrations that the author committed and the one that the script generated after running migrations. This check validates that everything was properly committed.


False positives

This job is allowed to fail, because it can throw some false positives.

For example, when we drop a column and then roll back, this column is always re-added at the end of the list of columns. If the column was previously in the middle of the list, the rollback can’t return the schema back exactly to its previous state. The job fails, but it’s an acceptable situation.

For a real-life example, refer to this failed job. Here, the author dropped the position column.

Schema dump comparison fails after rollback

This failure often happens if your working branch is behind the target branch. A real scenario:

graph LR Main((main<br>commit A)) ===> |remove constraint<br>fk_rails_dbebdaa8fe| MainB((main<br>commit B)) Main((main<br>commit A)) --> |checkout<br>dev| DevA((dev<br>commit A)):::dev DevA((dev<br>commit A)) --> |add column<br>dependency_proxy_size| DevC((dev<br>commit C)):::dev DevC -.-> |CI pipeline<br>executes| JOB-FAILED((JOB FAILED!)):::error classDef main fill:#f4f0ff,stroke:#7b58cf classDef dev fill:#e9f3fc,stroke:#1f75cb classDef error fill:#f15146,stroke:#d4121a
  1. You check out the dev working branch from the main target branch. At this point, each branch has their HEAD at commit A.
  2. Someone works on the main branch and drops the fk_rails_dbebdaa8fe constraint, thus creating commit B on main.
  3. You add column dependency_proxy_size to your dev branch.
  4. The db:check-migrations job fails for your dev branch’s CI/CD pipeline, because the structure.sql file did not rollback to its expected state.

This happened because branch dev contained commits A and C, not B. Its database schema did not know about the removal of the fk_rails_dbebdaa8fe constraint. When comparing the two schemas, the dev branch contained this constraint while the main branch didn’t.

This example really happened. Read the job failure logs.

To fix these kind of issues, rebase the working branch onto the target branch to get the latest changes.