ActiveRecord migrations outside of Rails

databases.rake

require 'yaml'
require 'logger'
require 'active_record'

namespace :db do
  def create_database config
    options = {:charset => 'utf8', :collation => 'utf8_unicode_ci'}

    create_db = lambda do |config|
      ActiveRecord::Base.establish_connection config.merge('database' => nil)
      ActiveRecord::Base.connection.create_database config['database'], options
      ActiveRecord::Base.establish_connection config
    end

    begin
      create_db.call config
    rescue Mysql::Error => sqlerr
      if sqlerr.errno == 1405
        print "#{sqlerr.error}. \nPlease provide the root password for your mysql installation\n>"
        root_password = $stdin.gets.strip

        grant_statement = <<-SQL
          GRANT ALL PRIVILEGES ON #{config['database']}.* 
            TO '#{config['username']}'@'localhost'
            IDENTIFIED BY '#{config['password']}' WITH GRANT OPTION;
        SQL

        create_db.call config.merge('database' => nil, 'username' => 'root', 'password' => root_password)
      else
        $stderr.puts sqlerr.error
        $stderr.puts "Couldn't create database for #{config.inspect}, charset: utf8, collation: utf8_unicode_ci"
        $stderr.puts "(if you set the charset manually, make sure you have a matching collation)" if config['charset']
      end
    end
  end
 
  task :environment do
    DATABASE_ENV = ENV['DATABASE_ENV'] || 'development'
    MIGRATIONS_DIR = ENV['MIGRATIONS_DIR'] || 'db/migrate'
  end

  task :configuration => :environment do
    @config = YAML.load_file('config/databases.yml')[DATABASE_ENV]
  end

  task :configure_connection => :configuration do
    ActiveRecord::Base.establish_connection @config
    ActiveRecord::Base.logger = Logger.new STDOUT if @config['logger']
  end

  desc 'Create the database from config/database.yml for the current DATABASE_ENV'
  task :create => :configure_connection do
    create_database @config
  end

  desc 'Drops the database for the current DATABASE_ENV'
  task :drop => :configure_connection do
    ActiveRecord::Base.connection.drop_database @config['database']
  end

  desc 'Migrate the database (options: VERSION=x, VERBOSE=false).'
  task :migrate => :configure_connection do
    ActiveRecord::Migration.verbose = true
    ActiveRecord::Migrator.migrate MIGRATIONS_DIR, ENV['VERSION'] ? ENV['VERSION'].to_i : nil
  end

  desc 'Rolls the schema back to the previous version (specify steps w/ STEP=n).'
  task :rollback => :configure_connection do
    step = ENV['STEP'] ? ENV['STEP'].to_i : 1
    ActiveRecord::Migrator.rollback MIGRATIONS_DIR, step
  end

  desc "Retrieves the current schema version number"
  task :version => :configure_connection do
    puts "Current version: #{ActiveRecord::Migrator.current_version}"
  end
end

Thanks for this gist! But let me correct you something: in https://gist.github.com/wbailey/822095#file-databases-rake-L43 should be database.yml instead of databases.yml

after setting up the configure file, type rake migrate. it complained "Don't know how to build task 'migrate'"
that is weird.

There is a better solution than defining the tasks yourself....
https://gist.github.com/drogus/6087979

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