Testing Rails migrations at GitLab

In order to reliably check Rails migrations, we need to test them against a database schema.

When to write a migration test

  • Post migrations (/db/post_migrate) and background migrations (lib/gitlab/background_migration) must have migration tests performed.
  • If your migration is a data migration then it must have a migration test.
  • Other migrations may have a migration test if necessary.

How does it work?

Adding a :migration tag to a test signature enables some custom RSpec before and after hooks in our spec/support/migration.rb to run.

A before hook reverts all migrations to the point that a migration under test is not yet migrated.

In other words, our custom RSpec hooks finds a previous migration, and migrate the database down to the previous migration version.

With this approach you can test a migration against a database schema.

An after hook migrates the database up and restores the latest schema version, so that the process does not affect subsequent specs and ensures proper isolation.

Testing an ActiveRecord::Migration class

To test an ActiveRecord::Migration class (i.e., a regular migration db/migrate or a post-migration db/post_migrate), you must load the migration file by using the require_migration! helper method because it is not autoloaded by Rails.

Example: 

require 'spec_helper'

require_migration!

RSpec.describe ...

Test helpers

require_migration!

Since the migration files are not autoloaded by Rails, you must manually load the migration file. To do so, you can use the require_migration! helper method which can automatically load the correct migration file based on the spec filename.

For example, if your spec file is named as populate_foo_column_spec.rb then the helper method tries to load ${schema_version}_populate_foo_column.rb migration file.

In case there is no pattern between your spec file and the actual migration file, you can provide the migration filename without the schema version, like so: 

require_migration!('populate_foo_column')

table

Use the table helper to create a temporary ActiveRecord::Base-derived model for a table. FactoryBot should not be used to create data for migration specs because it relies on application code which can change after the migration has run, and cause the test to fail. For example, to create a record in the projects table: 

project = table(:projects).create!(id: 1, name: 'gitlab1', path: 'gitlab1')

migrate!

Use the migrate! helper to run the migration that is under test. It runs the migration and bumps the schema version in the schema_migrations table. It is necessary because in the after hook we trigger the rest of the migrations, and we need to know where to start. Example: 

it 'migrates successfully' do
  # ... pre-migration expectations

  migrate!

  # ... post-migration expectations
end

reversible_migration

Use the reversible_migration helper to test migrations with either a change or both up and down hooks. This tests that the state of the application and its data after the migration becomes reversed is the same as it was before the migration ran in the first place. The helper:

  1. Runs the before expectations before the up migration.
  2. Migrates up.
  3. Runs the after expectations.
  4. Migrates down.
  5. Runs the before expectations a second time.

Example: 

reversible_migration do |migration|
  migration.before -> {
    # ... pre-migration expectations
  }

  migration.after -> {
    # ... post-migration expectations
  }
end

Example database migration test

This spec tests the db/post_migrate/20170526185842_migrate_pipeline_stages.rb migration. You can find the complete spec in spec/migrations/migrate_pipeline_stages_spec.rb. 

require 'spec_helper'

require_migration!

RSpec.describe MigratePipelineStages do
  # Create test data - pipeline and CI/CD jobs.
  let(:jobs) { table(:ci_builds) }
  let(:stages) { table(:ci_stages) }
  let(:pipelines) { table(:ci_pipelines) }
  let(:projects) { table(:projects) }

  before do
    projects.create!(id: 123, name: 'gitlab1', path: 'gitlab1')
    pipelines.create!(id: 1, project_id: 123, ref: 'master', sha: 'adf43c3a')
    jobs.create!(id: 1, commit_id: 1, project_id: 123, stage_idx: 2, stage: 'build')
    jobs.create!(id: 2, commit_id: 1, project_id: 123, stage_idx: 1, stage: 'test')
  end

  # Test just the up migration.
  it 'correctly migrates pipeline stages' do
    expect(stages.count).to be_zero

    migrate!

    expect(stages.count).to eq 2
    expect(stages.all.pluck(:name)).to match_array %w[test build]
  end

  # Test a reversible migration.
  it 'correctly migrates up and down pipeline stages' do
    reversible_migration do |migration|
      # Expectations will run before the up migration,
      # and then again after the down migration
      migration.before -> {
        expect(stages.count).to be_zero
      }

      # Expectations will run after the up migration.
      migration.after -> {
        expect(stages.count).to eq 2
        expect(stages.all.pluck(:name)).to match_array %w[test build]
      }
    end
end

Testing a non-ActiveRecord::Migration class

To test a non-ActiveRecord::Migration test (a background migration), you must manually provide a required schema version. Please add a schema tag to a context that you want to switch the database schema within.

If not set, schema defaults to :latest.

Example: 

describe SomeClass, schema: 20170608152748 do
  # ...
end

Example background migration test

This spec tests the lib/gitlab/background_migration/archive_legacy_traces.rb background migration. You can find the complete spec on spec/lib/gitlab/background_migration/archive_legacy_traces_spec.rb 

require 'spec_helper'

describe Gitlab::BackgroundMigration::ArchiveLegacyTraces, schema: 20180529152628 do
  include TraceHelpers

  let(:namespaces) { table(:namespaces) }
  let(:projects) { table(:projects) }
  let(:builds) { table(:ci_builds) }
  let(:job_artifacts) { table(:ci_job_artifacts) }

  before do
    namespaces.create!(id: 123, name: 'gitlab1', path: 'gitlab1')
    projects.create!(id: 123, name: 'gitlab1', path: 'gitlab1', namespace_id: 123)
    @build = builds.create!(id: 1, project_id: 123, status: 'success', type: 'Ci::Build')
  end

  context 'when trace file exists at the right place' do
    before do
      create_legacy_trace(@build, 'trace in file')
    end

    it 'correctly archive legacy traces' do
      expect(job_artifacts.count).to eq(0)
      expect(File.exist?(legacy_trace_path(@build))).to be_truthy

      described_class.new.perform(1, 1)

      expect(job_artifacts.count).to eq(1)
      expect(File.exist?(legacy_trace_path(@build))).to be_falsy
      expect(File.read(archived_trace_path(job_artifacts.first))).to eq('trace in file')
    end
  end
end

These tests do not run within a database transaction, as we use a deletion database cleanup strategy. Do not depend on a transaction being present.