Sometimes we want to query rails database outside of a rails app. But should you always bring AR for this? No, lightweight Sequel can handle this job pretty well. Check the guide Querying in Sequel to understand basics.

1) Add to Gemfile gems pg and sequel:

gem 'pg'
gem 'sequel'

2) Create module DB which contains AR models which you want to query. Sequel models have very similar syntax like ActiveRecord:

require 'pg'
require 'sequel'

Sequel.default_timezone = :utc

module DB
  URL = Sequel.connect("postgres://user:password@localhost/database_name")

  class Product < Sequel::Model(URL)
    plugin :timestamps, update_on_create: true
    many_to_one :brand
  end

  class Brand < Sequel::Model(URL)
    plugin :timestamps, update_on_create: true
    one_to_many :products
  end
end

This is it. Lets dig into details:

Sequel.default_timezone = :utc

Rails stores all dates in database using UTC format, where Sequel uses local time. We have to tell Sequel to use UTC instead.

URL = Sequel.connect("postgres://user:password@localhost/database_name")

This is a DATABASE_URL, format same as in Active Record. It is a good idea to put this string to .env file using Dotenv and then: URL = Sequel.connect(ENV["DATABASE_URL"]).

plugin :timestamps, update_on_create: true

If you want to create or update models as well, timestamps plugin will handle created_at and updated_at exact the same way as ActiveRecord.

Relations, where one_to_many equal to AR’s belongs_to and many_to_one equal to AR’s has_many. Read about Sequel relations more here.

And now you have everything to start:

product = DB::Product.first
product.brand
# etc.

There is full Sequel documentation http://sequel.jeremyevans.net/documentation.html.

Also, Sequel works with threads a bit simpler than ActiveRecord. If you ever tried to query AR inside threads, you know that connection pool can easy exceed and AR will raise an exception. That’s why in such cases all code which works with AR inside of threads should be wrapped to

ActiveRecord::Base.connection_pool.with_connection do
  # Do stuff
end

block.

Sequel does this automatically for you. You don’t have to do anything, and this is great. Read about it more here: Why you should stop using ActiveRecord and start using Sequel (“Threading” part).