This is the example contents of the Rakefile, which you would use to run active record tasks without using Rails. It assumes using the same directories as rails uses: `db/migrate`, `config/database.yml`.

Rakefile.rb

require 'bundler/setup'

require 'active_record'

include ActiveRecord::Tasks

db_dir = File.expand_path('../db', __FILE__)
config_dir = File.expand_path('../config', __FILE__)

DatabaseTasks.env = ENV['ENV'] || 'development'
DatabaseTasks.db_dir = db_dir
DatabaseTasks.database_configuration = YAML.load(File.read(File.join(config_dir, 'database.yml')))
DatabaseTasks.migrations_paths = File.join(db_dir, 'migrate')

task :environment do
  ActiveRecord::Base.configurations = DatabaseTasks.database_configuration
  ActiveRecord::Base.establish_connection DatabaseTasks.env
end

load 'active_record/railties/databases.rake'

If someone need only part of AR tasks (for example while testing gem you don't need half of them, because you test against test db and do not need seed or bunch of others) and for more details on how @drogus gist works, check my variation (based on this, thanks, btw :) https://gist.github.com/vanburg/56c5691c799c9e62e81a

Super helpful gist!

I'm using Ruby 2.3.1 and found I needed to change the env variable to a symbol when making the connection:

ActiveRecord::Base.establish_connection DatabaseTasks.env.to_sym

Otherwise I was getting a missing adapter error.

I would like to point to Jason modification of this gist, which works for ActiveRecord 4.2.0

Solved as:

# Rakefile
if Rake.application.top_level_tasks != ['test']
  require_relative 'config/application'
  Rails.application.load_tasks
  Rake::Task['test'].clear
end

task :test do
  ENV['RAILS_ENV'] = 'test'
  Dir.glob('./test/**/*_test.rb').each { |file| require file}
end

I have made minimal example/setup using Rails migration and active record outside Rails
https://github.com/euclid1990/rails-migration
(Support Rails >= 5.2)
🔢

I get
ActiveRecord::AdapterNotSpecified: database configuration does not specify adapter