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'

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