MigrationData

This gem provides functionality to write any code in migrations safely without regression.

Sometimes we have to write some Rails code in the migrations and it's hard to keep them in a working state because models which are used there change too often. There are some techniques which help to avoid these pitfalls. For example, define model classes in the migrations or write raw SQL. But they don't help in 100% of all cases. This gem promises to solve this problem in a simple way.

In short, this gem promotes writing the code migrates data in separate methods of a schema migration. That separates concerns and allows writing tests for these methods. Also, they optionally can be skipped.

If it's still not clear what this gem is for please check out this blog post.

Installation

Add this line to your application's Gemfile:

gem 'migration_data'

And then execute:

$ bundle

Or install it yourself as:

$ gem install migration_data

Usage

In your migration define a #data method:

class CreateUsers < ActiveRecord::Migration
  def change
    # Database schema changes as usual
  end

  def data
    User.create!(name: 'Andrey', email: 'ka8725@gmail.com')
  end

  def rollback
    User.find_by(name: 'Andrey', email: 'ka8725@gmail.com').destroy
  end
end

Now when migrations run with rake db:migrate command the #data method is executed right after the standard #change or #up method.

When migrations roll back with rake db:rollback command the #rollback is executed right after the standard #change or #down method.

It might appear that a data migration should run before the standard #change, #up, and #down methods. Define #data_before and #rollback_before methods for "up" and "down" directions correspondingly. There are also #data_after and #rollback_after methods for symmetry in that case, but basically they play the same role as #data and #rollback methods.

All these methods can be defined in one migration. They are executed in the following order when migration is run on up:

#data_before
#change/up
#data
#data_after

and on down:

#rollback_before
#change/down
#rollback
#rollback_after.

Note: in some circumstances, the reset_column_information method should be called on a model which table is changed in the migration. Especially when you are certain that there should be present some column for a model but it's absent for some reason. Read more about this in the official Rails docs.

Skipping data migrations execution

At some point, one might realize that data migrations should not run on particular environments, e.g. test.

On performing migrations in test environment, a data migration might try to add the same data that has already been added by seeds. In that case, migrations might fail with a duplication error.

Use MigrationData.config.skip = true to skip data migrations execution. One might put this code in an initializer, e.g. config/initializers/migration_data.rb:

if Rails.env.test?
  MigrationData.config.skip = true
end

Testing migrations

To keep your migrations working don't forget to write tests for them. It's preferably to put the tests for migrations into spec/db/migrations folder, but actually it's up to you. Possible RSpec test (spec/db/migrations/create_user.rb) for the migration looks like this:

require 'spec_helper'
require 'migration_data/testing'
require_migration 'create_users'

describe CreateUsers do
  describe '#data' do
    it 'works' do
      expect { described_class.new.data }.to_not raise_exception
    end
  end

  describe '#rollback' do
    before do
      described_class.new.data
    end

    it 'works' do
      expect { described_class.new.rollback }.to_not raise_exception
    end
  end
end

The helper to load migrations require_migration is defined in the migration_data/testing. So you should to require it to have access to this convenient require extension.