database.yml

test:
  adapter: postgresql
  encoding: unicode
  host: localhost
  database: gem_test
  username:
  password:
  pool: 5

Rakefile_with_custom_ar_rake_tasks_import.rb

require 'bundler/gem_tasks'
require 'active_record'

# Temporary switch to blank rake app, extract required tasks and import them to
# current rake task. Skip seed loader here, as we do not need it for tests.
# More info on AR tasks import: @see https://gist.github.com/drogus/6087979

def import_active_record_tasks(default_rake_app)
  Rake.application = Rake::Application.new
  Rake.application.rake_require('active_record/railties/databases')

  # Customize AR database tasks

  include ActiveRecord::Tasks

  db_dir                               = File.expand_path('../spec/db', __FILE__)
  db_config_path                       = db_dir + '/database.yml'
  migrations_path                      = db_dir + '/migrate'

  # WARNING! This MUST be a String not a Symbol
  DatabaseTasks.env                    = 'test'
  DatabaseTasks.db_dir                 = db_dir
  DatabaseTasks.database_configuration = YAML.load_file(db_config_path)
  DatabaseTasks.migrations_paths       = [migrations_path]

  # Several AR tasks rely (but do not include) on this task internally (like :create)
  # Here we establish connection. The first line is actually exists in :load_config
  # which is called AFTER :environment. And because we have custom :environment
  # task and want to connect, we copy that first line here.
  # Not the best solution, but don't have time dig deeper
  Rake::Task.define_task(:environment) do
    ActiveRecord::Base.configurations = DatabaseTasks.database_configuration
    # Use Symbol or you'll get deprecation warning
    ActiveRecord::Base.establish_connection(DatabaseTasks.env.to_sym)
  end

  tasks_to_import = %w[db:create db:drop db:purge db:rollback db:migrate
    db:migrate:up db:migrate:down db:migrate:status db:version db:schema:load]

  imported_tasks   = Rake.application.tasks.select do |task|
    tasks_to_import.include?(task.name)
  end

  # Restore default rake app
  Rake.application = default_rake_app

  # NOTE: didn't found a way to just use something like this tasks.define(task)
  imported_tasks.each do |task|
    # import description
    Rake.application.last_description = task.comment
    # import task
    Rake::Task.define_task(task.name) { task.invoke }
  end
end

import_active_record_tasks(Rake.application)

Hi Ivan, thank you for putting this together. Saves me lot of time in research. In our case we need to namespace active record tasks since we already have them defined for sequel. We are merging another app developed in rails/active record into our app which is in cuba/sequel.

Any suggestions on name spacing active record tasks so that we can do something like:

rake db:migrate:
execute sequel migrations
invoke active record migration task

@murtyk are you still looking for a solution? this might help: http://www.codeotaku.com/journal/2016-12/cool-things-about-rakefiles/index