Lappen

Rationale

class ProductsController < ApplicationController
  def index
    @products = policy_scope(Product.active).with_name(params[:name]).ordered_by(params[:order]).page(params[:page]).per(params[:per]).includes(:reviews).decorate
  end
end

Ever faced code like this? Right, me too. And I don't like it.

This Library targets the problem of Relation filtering (and polluted controller index actions) by abstracting the method chaining in a Pipeline-oriented fashion.

Supported Versions

• Ruby 2.2, 2.3
• Rails 4.0, 4.1, 4.2, 5.0

Installation

Add this line to your application's Gemfile:

gem 'lappen', github: 'tbuehlmann/lappen'

And then execute:

$ bundle install

Optionally create an ApplicationPipeline from which all Pipelines can inherit from by running:

$ bin/rails generate lappen:install

Usage

Defining a Pipeline

Inside your Rails application, define a subclass of Lappen::Pipeline for a Model you want to run Filters on. Inside this Pipeline, use the class method use to configure the Filters that shall be used:

class ProductPipeline < Lappen::Pipeline
  use Lappen::Filters::Kaminari
  use Lappen::Filters::Orderer
end

This Pipeline will use the Filters Kaminari and Orderer (in exactly this order).

A suitable location to place Pipelines is app/pipelines. This is also the default location for Pipeline created via the generators.

If you want to create a Pipeline for a specific Model, use the pipeline generator:

$ bin/rails generate lappen:pipeline product

Using a Pipeline

After defining a Pipeline, you can use it in your controller:

class ProductsController < ApplicationController
  def index
    @products = ProductPipeline.perform(Product.all, params)
  end
end

ProductPipeline.perform will apply any Filter you configured in the Pipeline class on the Relation you provided as the first argument. In order to do their work, Filters get access to the params object you can provide as the second argument.

A list of built-in Filters is available in the Wiki.

Shorthand Method

There's a shorthand method for finding and performing a Pipeline for a given Model or Relation. In order to enable the shorthand method, explicitly require lappen/scope in your Gemfile:

gem 'lappen', github: 'tbuehlmann/lappen', require: 'lappen/scope'

With that, having a Model Product with a Pipeline ProductPipeline, you can simply call Product.pipeline(params). This works internally by appending "Pipeline" to the calling Model's name. If you want to use the shorthand with a different Pipeline, define a class method pipeline_class on the Model:

class Product < ApplicationRecord
  class << self
    def pipeline_class
      MyProductPipeline
    end
  end
end

The pipeline method is defined on ApplicationRecord if existing or ActiveRecord::Base otherwise.

Writing a Filter

If you want to write and use your own Filter, create a class following the common interface:

class MyFilter
  def initialize(*args, **options)
    # …
  end

  def perform(scope, params = {})
    # …
  end
end

… and add the Filter to your Pipeline:

class ProductPipeline < Lappen::Pipeline
  use Lappen::Filters::Kaminari
  use Lappen::Filters::Orderer
  use MyFilter
end

Any further arguments are injected into the Filter's constructor:

  use MyFilter, :foo, bar: 'baz'

… will initialize the Filter as MyFilter.new(:foo, bar: 'baz').

In order to save you some time, there's already a class following the interface you can inherit from:

class MyFilter < Lappen::Filter
  # …
end

Inside the class you have access to args and options getter methods referencing the initialized arguments.

Returning Early

Normally, the Pipeline runs each Filter after another, using a filter's output as the next filter's input. If you want to stop further filtering (for whatever reason), throw :halt with the result scope:

class MyFilter < Lappen::Filter
  def perform(scope, params = {})
    if some_condition
      throw(:halt, scope.none)
    else
      # …
    end
  end
end

Throwing :halt will stop the Pipeline from running any further Filter and the throwed scope will be returned.

Request Context

There are times in life you need to use information from the current request in your Filter. In order to do so, Lappen::Filter includes the Lappen::RequestContext module, which enables you to use the private controller and view_context methods. Using those you can for example access the commonly used current_user method:

class MyFilter < Lappen::Filter
  def perform(scope, params = {})
    # …
  end

  private

  def current_user
    view_context.current_user
  end
end

Callbacks

When including Lappen::Callbacks into a Pipeline or Filter, it will support (before/around/after) Callbacks in order to be able to hook into its perform action.

Example:

class ProductPipeline < Lappen::Pipeline
  include Lappen::Callbacks

  before_perform { puts 'Pipeline.before_perform' }
  after_perform  { puts 'Pipeline.after_perform' }

  around_perform do |pipeline, block|
    puts 'Pipeline.around_perform (before)'
    block.call
    puts 'Pipeline.around_perform (after)'
  end

  use Lappen::Filters::Equal
end

class Lappen::Filters::Equal
  include Lappen::Callbacks

  before_perform { puts 'Filter.before_perform' }
  after_perform  { puts 'Filter.after_perform' }

  around_perform do |filter, block|
    puts 'Filter.around_perform (before)'
    block.call
    puts 'Filter.around_perform (after)'
  end
end

ProductPipeline.perform(scope = {})

# Pipeline.before_perform
# Pipeline.around_perform (before)
# Filter.before_perform
# Filter.around_perform (before)
# Filter.around_perform (after)
# Filter.after_perform
# Pipeline.around_perform (after)
# Pipeline.after_perform

Callbacks are available in subclasses dynamically. Defining a Callback in a superclass will make it available in its subclasses, even if they were already defined.

Notifications

If you want to fanout ActiveSupport::Notifications on the performings of a Pipeline or Filter, include the Lappen::Notifications module into your Pipeline or Filter:

class ProductPipeline < Lappen::Pipeline
  include Lappen::Notifications
end

class Lappen::Filters::Equal
  include Lappen::Notifications
end

By doing so, a Notification Event with the key 'lappen.pipeline.perform' (or 'lappen.filter.perform') will be sent. You can subscribe to the Events like this:

ActiveSupport::Notifications.subscribe('lappen.pipeline.perform') do |name, start, finish, id, payload|
  puts payload[:pipeline]
end

ActiveSupport::Notifications.subscribe('lappen.filter.perform') do |name, start, finish, id, payload|
  puts payload[:filter]
end

As Lappen::Notifications uses Callbacks internally, subclasses of a class that has the Lappen::Notifications module included will also instrument Notifications.

Meta Information

It can be comfortable to know which Filter applied what condition to the scope. Consider an API that, beside delivering resources, provides information about attribute filtering, pagination or included associated resources. Meta information give you exactly this.

Each Filter has access to a meta object which is used to memoize the conditions applied to a scope by a Pipeline's Filters. The meta object is a Hash that gets populated by each Filter that is run. The Equal Filter for example would do something like the following when filtering the price:

meta[:equal] ||= {}
meta[:equal].merge!(price: 5000)

The meta object is accessable as scope.meta. Example:

class ProductPipeline < Lappen::Pipeline
  use Lappen::Filters::Equal, :price
end

products = ProductPipeline.perform(Product.all, {filter: {price: 5000}})
products.meta # => {equal: {price: 5000}}