Forcing a specific version?

[jf-uu]

golang-migrate has support for forcing a specific version via migrate force <N>. This is useful when creating migrations for an already existing schema, as you can create a migration representing the current state and force the version to that (as it techincally already is).

Manually creating and setting the version in the schema version table is one option, but a force command would be nice.

[pscheid92]

The main use case for force is baselining an existing database when adopting tern, right?

You write a migration that represents the current schema, but you can't run it because the objects already exist.

Sure, using idempotent DDL (CREATE TABLE IF NOT EXISTS, etc.) in the baseline migration is a reasonable default, but it doesn't cover all cases. Not all DDL supports IF NOT EXISTS (for instance CREATE TYPE or some constraint/index variants).

Today, the only option, as @jf-uu mentioned, is inserting the version into the table manually. So, I see how a tern force <version> command would make this an official, documented workflow.

If this suits your imagination @jackc I happily volunteer.

[jackc]

I suppose that's reasonable. Only real question would be the interface.

[pscheid92]

Ok, this is how I imagine using the CLI. I moved away from force to set-version to be more explicit about what is happening.

Success:
$ tern set-version 5
version set: 3 → 5

Already at that version:
$ tern set-version 5
version set: 5 → 5

Connection error:
$ tern set-version 5
Unable to connect to PostgreSQL:
  connection refused

Invalid argument:
$ tern set-version abc
Error: "abc" is not a valid version number

Negative version:
$ tern set-version -1
Error: version must be non-negative

Missing argument:
$ tern set-version
Error: accepts 1 arg(s), received 0
Usage:
  tern set-version VERSION [flags]

Migrator method would look like this:

func (m *Migrator) SetVersion(ctx context.Context, version int32) error {
    err := acquireAdvisoryLock(ctx, m.conn)
    if err != nil {
        return err
    }
    defer func() {
        unlockErr := releaseAdvisoryLock(ctx, m.conn)
        if err == nil && unlockErr != nil {
            err = unlockErr
        }
    }()

    _, err = m.conn.Exec(ctx, "update "+m.versionTable+" set version=$1", version)
    return err
}

[jackc]

Usage looks fine. set-version is acceptable as command name, but I wonder if it could be made even more clear that this is just setting the version number and not actually running any migrations. Have you considered something like override-version or reset-version?

[pscheid92]

I would second override-version as it conveys the danger of this operation way better than set-version! The variant reset-version implies resetting to some kind of zero or base value, which is misleading.

I'll prepare an MR with override-version. Changing it during PR review should be easy.

[jf-uu]

Thank you both!