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'

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