Add an flag for skipping the schema version check

If you chose to use the rollback migration feature on your current
version for example, you should still have a way to migrate, being
that you are still on a supported migration path.

.gitlab/ci/rails.gitlab-ci.yml

@@ -244,7 +244,7 @@ migration:path-pg:
   extends: .dedicated-no-docs-and-no-qa-pull-cache-job
   script:
     - bundle exec rake db:migrate VERSION=20170523121229
-    - bundle exec rake db:migrate
+    - bundle exec rake db:migrate SKIP_SCHEMA_VERSION_CHECK=true
   dependencies:
     - setup-test-env
 

changelogs/unreleased/check-min-schema-migrate.yml

 ---
-title: Added a min schema version check to gitlab:db:configure
+title: Added a min schema version check to db:migrate
 merge_request: 29882
 author:
 type: added

lib/tasks/migrate/schema_check.rake

 desc 'Configures the database by running migrate, or by loading the schema and seeding if needed'
 task schema_version_check: :environment do
+  next if ENV['SKIP_SCHEMA_VERSION_CHECK']
+
   schema_version = ActiveRecord::Migrator.current_version
 
   # Ensure migrations are being run from a supported schema version

spec/tasks/gitlab/db_rake_spec.rb

@@ -16,7 +16,6 @@ describe 'gitlab:db namespace rake task' do
     allow(Rake::Task['db:migrate']).to receive(:invoke).and_return(true)
     allow(Rake::Task['db:schema:load']).to receive(:invoke).and_return(true)
     allow(Rake::Task['db:seed_fu']).to receive(:invoke).and_return(true)
-    allow(ActiveRecord::Migrator).to receive(:current_version).and_return(Gitlab::Database::MIN_SCHEMA_VERSION)
   end
 
   describe 'configure' do
@@ -28,12 +27,6 @@ describe 'gitlab:db namespace rake task' do
       expect { run_rake_task('gitlab:db:configure') }.not_to raise_error
     end
 
-    it 'raises an when schema has been loaded, but version is too old to migrate' do
-      allow(ActiveRecord::Base.connection).to receive(:tables).and_return(%w[table1 table2])
-      allow(ActiveRecord::Migrator).to receive(:current_version).and_return(25)
-      expect { run_rake_task('gitlab:db:configure') }.to raise_error(RuntimeError, /current database version is too old to be migrated/)
-    end
-
     it 'invokes db:shema:load and db:seed_fu when schema is not loaded' do
       allow(ActiveRecord::Base.connection).to receive(:tables).and_return([])
       expect(Rake::Task['db:schema:load']).to receive(:invoke)

spec/tasks/migrate/schema_check_rake_spec.rb

@@ -2,8 +2,11 @@ require 'spec_helper'
 require 'rake'
 
 describe 'schema_version_check rake task' do
+  include StubENV
+
   before :all do
     Rake.application.rake_require 'active_record/railties/databases'
+    Rake.application.rake_require 'tasks/migrate/schema_check'
 
     # empty task as env is already loaded
     Rake::Task.define_task :environment
@@ -13,6 +16,13 @@ describe 'schema_version_check rake task' do
     # Stub out db tasks
     allow(ActiveRecord::Tasks::DatabaseTasks).to receive(:migrate).and_return(true)
     allow(ActiveRecord::Migrator).to receive(:current_version).and_return(Gitlab::Database::MIN_SCHEMA_VERSION)
+
+    # Ensure our check can re-run each time
+    Rake::Task[:schema_version_check].reenable
+  end
+
+  it 'allows migrations on databases meeting the min schema version requirement' do
+    expect { run_rake_task('db:migrate') }.not_to raise_error
   end
 
   it 'raises an error when schema version is too old to migrate' do
@@ -20,6 +30,17 @@ describe 'schema_version_check rake task' do
     expect { run_rake_task('db:migrate') }.to raise_error(RuntimeError, /current database version is too old to be migrated/)
   end
 
+  it 'skips running validation when passed the skip env variable' do
+    stub_env('SKIP_SCHEMA_VERSION_CHECK', 'true')
+    allow(ActiveRecord::Migrator).to receive(:current_version).and_return(25)
+    expect { run_rake_task('db:migrate') }.not_to raise_error
+  end
+
+  it 'allows migrations on fresh databases' do
+    allow(ActiveRecord::Migrator).to receive(:current_version).and_return(0)
+    expect { run_rake_task('db:migrate') }.not_to raise_error
+  end
+
   def run_rake_task(task_name)
     Rake::Task[task_name].reenable
     Rake.application.invoke_task task_name