ranked-model is a modern row sorting library built for Rails 4.2+. It uses ARel aggressively and is better optimized than most other libraries.

ANNOUNCING: Positioning, the gem

As maintainer of both Acts As List and the Ranked Model gems, I've become intimately aquainted with the strengths and weaknesses of each. I ended up writing a small scale Rails Concern for positioning database rows for a recent project and it worked really well so I've decided to release it as a gem: Positioning

Positioning works similarly to Acts As List in that it maintains a sequential list of integer values as positions. It differs in that it encourages a unique constraints on the position column and supports multiple lists per database table. It borrows Ranked Model's concept of relative positioning. I encourage you to check it out and give it a whirl on your project!

Installation

ranked-model passes specs with Rails 4.2, 5.0, 5.1, 5.2, 6.0 and 6.1 for MySQL, Postgres, and SQLite on Ruby 2.4 through 3.0 (with exceptions, please check the CI setup for supported combinations), and jruby-9.1.17.0 where Rails supports the platform.

To install ranked-model, just add it to your Gemfile:

gem 'ranked-model'

# Or pin ranked-model to git
# gem 'ranked-model',
#   git: 'git@github.com:mixonic/ranked-model.git'

Then use bundle install to update your Gemfile.lock.

Simple Use

Use of ranked-model is straight ahead. Get some ducks:

class Duck < ActiveRecord::Base
end

Give them an order (integer column):

rails g migration AddRowOrderToDucks row_order:integer
rails db:migrate

IMPORTANT: The _order table column MUST allow null values. For the reason behind this requirement see issue#167

Put your ducks in a row:

class Duck < ActiveRecord::Base

  include RankedModel
  ranks :row_order

end

Order the Ducks by this order:

Duck.rank(:row_order).all

The ranking integers stored in the row_order column will be big and spaced apart. When you implement a sorting UI, just update the resource by appending the column name with _position and indicating the desired position:

@duck.update row_order_position: 0  # or 1, 2, 37. :first, :last, :up and :down are also valid

IMPORTANT: Note that you MUST append _position to the column name when setting a new position on an instance. This is a fake column that can take relative as well as absolute index-based values for position.

Position numbers begin at zero. A position number greater than the number of records acts the same as :last. :up and :down move the record up/down the ladder by one step.

So using a normal json controller where @duck.attributes = params[:duck]; @duck.save, JS can look pretty elegant:

$.ajax({
  type: 'PUT',
  url: '/ducks',
  dataType: 'json',
  data: { duck: { row_order_position: 0 } },  // or whatever your new position is
});

If you need to find the rank of an item with respect to other ranked items, you can use the {column_name}_rank method on the model instance. {column_name} is your resource ranking column.

Following on from our examples above, the row_order_rank method will return the position of the duck object in the list with respect to the order defined by the row_order column.

Duck.rank(:row_order).first.row_order_rank # => 0
Duck.rank(:row_order).third.row_order_rank # => 2

Complex Use

The ranks method takes several arguments:

class Duck < ActiveRecord::Base

  include RankedModel

  ranks :row_order,                  # Name this ranker, used with rank()
    column: :sort_order              # Override the default column, which defaults to the name

  belongs_to :pond
  ranks :swimming_order,
    with_same: :pond_id              # Ducks belong_to Ponds, make the ranker scoped to one pond

  ranks :row_order,
    with_same: [:pond_id, :breed]    # Lets rank them by breed

  scope :walking, where(walking: true )
  ranks :walking_order,
    scope: :walking                  # Narrow this ranker to a scope

  belongs_to :parent, class_name: 'Duck', optional: true
  ranks :child_order,
    unless: :has_no_parent?,         # Rank only ducks that have a parent. Alternatively a Proc or lambda can be passed, e.g. proc { parent.nil? }
    with_same: :parent_id

  def has_no_parent?
    parent.nil?
  end
end

When you make a query, add the rank:

Duck.rank(:row_order)

Pond.first.ducks.rank(:swimming_order)

Duck.walking.rank(:walking)

Drawbacks

While ranked-model is performant when storing data, it might cause N+1s depending on how you write your code. Consider this snippet:

ducks = Duck.all
ducks.map do |duck|
  {
    id: duck.id,
    position: duck.row_order_rank # This causes N+1!
  }
end

Every call to duck.row_order_rank will make a call to the DB to check the rank of that particular element. If you have a long list of elements this might cause issues to your DB.

In order to avoid that, you can use the rank(:your_rank) scope and some Ruby code to get the element's position:

ducks = Duck.rank(:row_order).all
ducks.map.with_index do |duck, index|
  {
    id: duck.id,
    position: index
  }
end

Single Table Inheritance (STI)

ranked-model scopes your records' positions based on the class name of the object. If you have a STI type column set in your model, ranked-model will reference that class for positioning.

Consider this example:

class Vehicle < ActiveRecord::Base
  ranks :row_order
end

class Car < Vehicle
end

class Truck < Vehicle
end

car = Car.create!
truck = Truck.create!

car.row_order
=> 0
truck.row_order
=> 0

In this example, the row_order for both car and truck will be set to 0 because they have different class names (Car and Truck, respectively).

If you would like for both car and truck to be ranked together based on the base Vehicle class instead, use the class_name option:

class Vehicle < ActiveRecord::Base
  ranks :row_order, class_name: 'Vehicle'
end

class Car < Vehicle
end

class Truck < Vehicle
end

car = Car.create!
truck = Truck.create!

car.row_order
=> 0
truck.row_order
=> 4194304

Migrations for existing data

If you use ranked_model with existing data, the following migration (for Rails 6) can be a starting point. Make sure to declare include RankedModel and ranks :row_order in your Duck before running the migration.

rails g migration AddRowOrderToDucks row_order:integer

Then, adjust the migration:

# e.g. file db/migrate/20200325095038_add_row_order_to_ducks.rb
class AddRowOrderToDucks < ActiveRecord::Migration[6.0]
  def change
    add_column :ducks, :row_order, :integer

    # Newest Duck shall rank "highest"" (be last).
    Duck.update_all('row_order = EXTRACT(EPOCH FROM created_at)')

    # Alternatively, implement any other sorting default
    # Duck.order(created_at: :desc).each do |duck|
    #   duck.update!(row_order: duck.created_at.to_i + duck.age / 2)
    # end
  end
end

Internals

This library is written using ARel from the ground-up. This leaves the code much cleaner than many implementations. ranked-model is also optimized to write to the database as little as possible: ranks are stored as a number between -2147483648 and 2147483647 (the INT range in MySQL). When an item is given a new position, it assigns itself a rank number between two neighbors. This allows several movements of items before no digits are available between two neighbors. When this occurs, ranked-model will try to shift other records out of the way. If items can't be easily shifted anymore, it will rebalance the distribution of rank numbers across all members of the ranked group.

Record updates to rebalance ranks do not trigger ActiveRecord callbacks. If you need to react to these updates (to index them in a secondary data store, for example), you can subscribe to the ranked_model.ranks_updated ActiveSupport notification. Subscribed consumers receive an event for each rearrangement or rebalancing, the payload of which includes the triggering instance and the scope and with_same options for the ranking, which can be used to retrieve the affected records.

ActiveSupport::Notifications.subscribe("ranked_model.ranks_updated") do |_name, _start, _finish, _id, payload|
  # payload[:instance] - the instance whose update triggered the rebalance
  # payload[:scope] - the scope applied to the ranking
  # payload[:with_same] - the with_same option applied to the ranking
end

Contributing

Fork, clone, write a test, write some code, commit, push, send a pull request. Github FTW!

The code is published under the MIT License.

The specs can be run with sqlite, postgres, and mysql:

bundle
appraisal install
DB=postgresql bundle exec appraisal rake

If no DB is specified (sqlite, mysql, or postgresql), the tests run against sqlite.

RankedModel is mostly the handiwork of Matthew Beale:

• madhatted.com is where I blog. Also @mixonic.

A hearty thanks to these contributors:

• Harvest where this Gem started. They are great, great folks.
• yabawock
• AndrewRadev
• adheerajkumar
• mikeycgto
• robotex82
• rociiu
• codepodu
• kakra
• metalon
• jamesalmond
• jguyon
• pehrlich
• petergoldstein
• brendon