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https://github.com/brightcommerce/attr_sequence

ActiveRecord concern that generates scoped sequential numbers for models.
https://github.com/brightcommerce/attr_sequence

activerecord concern macro ruby-on-rails sequencer

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ActiveRecord concern that generates scoped sequential numbers for models.

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# AttrSequence



AttrSequence is an ActiveRecord concern that generates scoped sequential numbers for models. This gem provides an `attr_sequence` macro that automatically assigns a unique, sequential number to each record. The sequential number is not a replacement for the database primary key, but rather adds another way to retrieve the object without exposing the primary key.



AttrSequence has been extracted from the Brightcommerce platform and is now used in multiple other software projects.



## Installation



To install add the line to your `Gemfile`:



``` ruby

gem 'attr_sequence'

```



And run `bundle install`.



The following configuration defaults are used by AttrSequence:



``` ruby

AttrSequence.configure do |config|

  config.column = :number

  config.start_at = 1

end

```



You can override them by generating an initializer using the following command:



``` bash

rails generate attr_sequence:initializer

```



This will generate an initializer file in your project's `config/initializers` called `attr_sequence.rb` directory.



## Usage



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 AttrSequence:



``` ruby

class Question < ActiveRecord::Base

  has_many :answers

end



class Answer < ActiveRecord::Base

  include AttrSequence

  belongs_to :question

  attr_sequence scope: :question_id

end

```



To autoload AttrSequence for all models, add the following to an initializer:



``` ruby

require 'attr_sequence/active_record'

```



You then don't need to `include AttrSequence` in any model.



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



``` bash

rails generate migration add_number_to_answers number:integer

rake db:migrate

```



Then, include the concern module and call the `attr_sequence` macro in your model class:



``` ruby

class Answer < ActiveRecord::Base

  include AttrSequence

  belongs_to :question

  attr_sequence 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:



``` ruby

class Answer < ActiveRecord::Base

  include AttrSequence

  belongs_to :questionable, polymorphic: true

  attr_sequence scope: [:questionable_id, :questionable_type]

end

```



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



``` ruby

class Answer < ActiveRecord::Base

  include AttrSequence

  belongs_to :account

  belongs_to :question



  attr_sequence column: :question_answer_number, scope: :question_id

  attr_sequence column: :account_answer_number, scope: :account_id

end

```



## Schema and data integrity



*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 number column (or a multicolumn unique index on sequential number and scope columns, if you are using scopes). This will ensure that you can never have duplicate sequential numbers 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 number column as well if you never intend to skip it.



Here is an example migration for an `Answer` model that has a `:number` scoped to a `Question`:



``` ruby

# app/db/migrations/20180101000000_create_answers.rb

class CreateAnswers < ActiveRecord::Migration

  def change

    create_table :answers do |table|

      table.references :question

      table.column :number, :integer, null: false

      table.index [:number, :question_id], unique: true

    end

  end

end

```



## Configuration



### Overriding the default sequential ID column



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



``` ruby

attr_sequence scope: :question_id, column: :my_sequential_id

```



### Starting the sequence at a specific number



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



``` ruby

attr_sequence start_at: 1000

```



You may also pass a lambda to the `start_at` option:



``` ruby

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

```



### Indexing the sequential number column



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



### Skipping sequential ID generation



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



``` ruby

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

```



## Example



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



``` ruby

# app/models/question.rb

class Question < ActiveRecord::Base

  has_many :answers

end



# app/models/answer.rb

class Answer < ActiveRecord::Base

  include AttrSequence

  belongs_to :question

  attr_sequence scope: :question_id



  # Automatically use the sequential number in URLs

  def to_param

    self.number.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(number: params[:id])

  end

end

```



Now, answers are accessible via their sequential numbers:



```

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

```



instead of by their primary keys:



```

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

```



## Dependencies



AttrSequence gem has the following runtime dependencies:

- activerecord >= 5.1.4

- activesupport >= 5.1.4



## Compatibility



Tested with MRI 2.4.2 against Rails 5.2.2.



## Contributing



1. Fork it

2. Create your feature branch (`git checkout -b my-new-feature`)

3. Commit your changes (`git commit -am 'Add some feature'`)

4. Push to the branch (`git push origin my-new-feature`)

5. Create new Pull Request



## Credit



This gem was written and is maintained by [Jurgen Jocubeit](https://github.com/JurgenJocubeit), CEO and President Brightcommerce, Inc.



## License



The gem is available as open source under the terms of the [MIT License](http://opensource.org/licenses/MIT).



## Copyright



Copyright 2018 Brightcommerce, Inc.