How temporarily disable "needs_migration?" check when testing migration?

Question

I've written the spec to test my migration but when I run it I got an error:

ActiveRecord::PendingMigrationError:

Migrations are pending. To resolve this issue, run:

    bin/rake db:migrate RAILS_ENV=test

I've tried to disable the migration check in the before section but that check is running before all tests.

How to disable the migration check for testing purposes?

Answer 1

Testing Rails migration is a bit of a pain so I would rather step back and think about if this needs to be in a Rails migration / tested in a Rails migration.

There are basically two different types of migrations

Schema migrations

Use mostly Rails built in functions. Unless you do some handcrafted SQL I wouldn't bother testing this and trust the framework here.

Data migrations

Data migrations are used to backfill or change data. As data is one of your most valuable assets and loosing or corrupting it is very painful I would definitely recommend to write tests for data migrations.

As mentioned, testing migrations is a bit of a pain so I would try to abstract the data migration code in it's own (service) class. Something like

class DataMigration::UpdateUsername
  def self.run
    new.run
  end

  def run
    User.all do |batch|
      user.update(name: user.name.capitalize)
    end
  end
end

You can now test the data migration like a normal class like this:

it 'does capitalize the name' do
  user = create(:user, name: 'name')

  DataMigration::UpdateUsername.run

  expect(user.reload.name).to eq('NAME')
end

Now we can use this class in our Rails migration or e.g. just use it in a Rake task. Using it in a Rake task also has the advantages that we can pass in parameters, run several data migrations in parallel (e.g. you have a large data set) or even in a background job which you can't in a Rails migration.

Example

class DataMigration::UpdateUsername
  def initialize(start_id:, finish_id:)
    @start_id = start_id
    @finish_id = finish_id
  end

  def run
    User.find_in_batches(start: start_id, finish: finish_id) do |batch|
      batch.each do |user|
        user.update(name: user.name.capitalize)
      end
    end
  end
end

Now we can create a custom task for this

namespace :db do
  desc "Runs user data migration"
  task :update_user, [:start, :finish] do |task, args|
    DataMigration::UpdateUsername.new(start_id: args[:start], finish_id: args[:finish])
  end
end

rake db:update_user[0, 10000]
rake db:update_user[10000, 20000]
# ...