Sequenced is a simple gem that generates scoped sequential IDs for ActiveRecord models. This gem provides an acts_as_sequenced macro that automatically assigns a unique, sequential ID to each record. The sequential ID is not a replacement for the database primary key, but rather adds another way to retrieve the object without exposing the primary key.

It's generally a bad practice to expose your primary keys to the world in your URLs. However, it is often appropriate to number objects in sequence (in the context of a parent object).

For example, given a Question model that has many Answers, it makes sense to number answers sequentially for each individual question. You can achieve this with Sequenced in one line of code:

class Question < ActiveRecord::Base
  has_many :answers
end

class Answer < ActiveRecord::Base
  belongs_to :question
  acts_as_sequenced scope: :question_id
end

Add the gem to your Gemfile:

gem 'sequenced'

Install the gem with bundler:

bundle install

To add a sequential ID to a model, first add an integer column called sequential_id to the model (or you many name the column anything you like and override the default). For example:

rails generate migration add_sequential_id_to_answers sequential_id:integer
rake db:migrate

Then, call the acts_as_sequenced macro in your model class:

class Answer < ActiveRecord::Base
  belongs_to :question
  acts_as_sequenced scope: :question_id
end

The scope option can be any attribute, but will typically be the foreign key of an associated parent object. You can even scope by multiple columns for polymorphic relationships:

class Answer < ActiveRecord::Base
  belongs_to :questionable, :polymorphic => true
  acts_as_sequenced scope: [:questionable_id, :questionable_type]
end

Multiple sequences can be defined by using the macro multiple times:

class Answer < ActiveRecord::Base
  belongs_to :account
  belongs_to :question

  acts_as_sequenced column: :question_answer_number, scope: :question_id
  acts_as_sequenced column: :account_answer_number, scope: :account_id
end

This gem is only concurrent-safe for PostgreSQL databases. For other database systems, unexpected behavior may occur if you attempt to create records concurrently.

You can mitigate this somewhat by applying a unique index to your sequential ID column (or a multicolumn unique index on sequential ID and scope columns, if you are using scopes). This will ensure that you can never have duplicate sequential IDs within a scope, causing concurrent updates to instead raise a uniqueness error at the database-level.

It is also a good idea to apply a not-null constraint to your sequential ID column as well if you never intend to skip it.

Here is an example migration for a model that has a sequential_id scoped to a burrow_id:

# app/db/migrations/20151120190645_create_badgers.rb
class CreateBadgers < ActiveRecord::Migration
  def change
    create_table :badgers do |t|
      t.integer :sequential_id, null: false
      t.integer :burrow_id
    end

    add_index :badgers, [:sequential_id, :burrow_id], unique: true
  end
end

If you are adding a sequenced column to an existing table, you need to account for that in your migration.

Here is an example migration that adds and sets the sequential_id column based on the current database records:

# app/db/migrations/20151120190645_add_sequental_id_to_badgers.rb
class AddSequentalIdToBadgers < ActiveRecord::Migration
  add_column :badgers, :sequential_id, :integer

  execute <<~SQL
    UPDATE badgers
    SET sequential_id = old_badgers.next_sequential_id
    FROM (
      SELECT id, ROW_NUMBER()
      OVER(
        PARTITION BY burrow_id
        ORDER BY id
      ) AS next_sequential_id
      FROM badgers
    ) old_badgers
    WHERE badgers.id = old_badgers.id
  SQL

  change_column :badgers, :sequential_id, :integer, null: false
  add_index :badgers, [:sequential_id, :burrow_id], unique: true
end

By default, Sequenced uses the sequential_id column and assumes it already exists. If you wish to store the sequential ID in different integer column, simply specify the column name with the column option:

acts_as_sequenced scope: :question_id, column: :my_sequential_id

By default, Sequenced begins sequences with 1. To start at a different integer, simply set the start_at option:

acts_as_sequenced start_at: 1000

You may also pass a lambda to the start_at option:

acts_as_sequenced start_at: lambda { |r| r.computed_start_value }

For optimal performance, it's a good idea to index the sequential ID column on sequenced models.

If you'd like to skip generating a sequential ID under certain conditions, you may pass a lambda to the skip option:

acts_as_sequenced skip: lambda { |r| r.score == 0 }

Suppose you have a question model that has many answers. This example demonstrates how to use Sequenced to enable access to the nested answer resource via its sequential ID.

# app/models/question.rb
class Question < ActiveRecord::Base
  has_many :answers
end

# app/models/answer.rb
class Answer < ActiveRecord::Base
  belongs_to :question
  acts_as_sequenced scope: :question_id

  # Automatically use the sequential ID in URLs
  def to_param
    self.sequential_id.to_s
  end
end

# config/routes.rb
resources :questions do
  resources :answers
end

# app/controllers/answers_controller.rb
class AnswersController < ApplicationController
  def show
    @question = Question.find(params[:question_id])
    @answer = @question.answers.find_by(sequential_id: params[:id])
  end
end

Now, answers are accessible via their sequential IDs:

http://example.com/questions/5/answers/1  # Good

instead of by their primary keys:

http://example.com/questions/5/answer/32454  # Bad

1. Fork it
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Added some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create new Pull Request