How do i work with two different databases in rails with active records?

Add new sections to your database.yml e.g.

other_development:
  adapter: mysql
  database: otherdb_development
  username: root
  password:
  host: localhost

other_production:
  adapter: mysql
  database: otherdb_production
  username: root
  password:
  host: localhost

Add a class in lib/other_database.rb

class OtherDatabase < ActiveRecord::Base
  establish_connection "other_#{RAILS_ENV}"
end

and then for each model which isn't in the default database subclass from OtherDatabase e.g.:

class MyModel < OtherDatabase
   # my model code...
end

mikej is right. I did however write a gem that makes the model code to connect a little bit cleaner, check it out.

Rails 6 added native support for multiple databases: https://edgeguides.rubyonrails.org/active_record_multiple_databases.html

database.yml

development:
  one:
    <<: *default
  other:
    <<: *default

Model base classes:

class OneModelBase < ActiveRecord::Base
  around_action :set_db

  private

  def set_db
    ActiveRecord::Base.connected_to(database: :one) do
      yield
    end
  end
end

class OtherModelBase < ActiveRecord::Base
  around_action :set_db

  private

  def set_db
    ActiveRecord::Base.connected_to(database: :other) do
      yield
    end
  end
end

You'll also have different migrations and schemas per DB. Running a command like rails db:create will create all databases. You can scope commands, e.g. rails db:create:other.

In rails 4.1+ establish_connection now takes a symbol:

class OtherDbModel < ActiveRecord::Base
  establish_connection :"other_#{Rails.env}"
end