Collection pattern

class LeadsController < ApplicationController
  def index
    collection = Collections::LeadsCollection.new(current_ability, current_user, params)
    @leads = collection.eager_loaded.paginated.items
  end
end

module Collections
  class Collection
    module Authorization
      def items
        super.accessible_by(ability)
      end
    end

    module Ordering
      def items
        if has_custom_ordering?
          super.reorder(ordering_sql)
        else
          super.sorted_by_date
        end
      end

      private

      def has_custom_ordering?
        order_key.present?
      end

      def ordering_sql
        keys = order_key.split(',')
        keys.map { |k| "#{k} #{order_dir}" }.join(',')
      end

      def order_key
        params[:order_key]
      end

      def order_dir
        params[:order_dir] =~ /^desc$/i ? 'desc' : 'asc'
      end
    end

    module Pagination
      def items
        super.page(params[:page]).per(params[:per])
      end
    end

    module Search
      def items
        super.search_for(params[:search])
      end
    end
  end
end

module Collections
  class LeadsCollection < Collection

    module Scope
      def items
        super
      end
    end

    module LeadsStatusScope
      def items
        super.in_status(params[:lead_status])
      end
    end

    module DepartmentScope
      def items
        department_ids.include?(params[:department]) ? super.with_department(params[:department]) : super
      end

      private

      def department_ids
        Deprtment.all.select(:id).map(&:id).map(&:to_s)
      end
    end

    module RoleScope
      def items
        params[:role].present? ? super.with_role(params[:role]) : super
      end
    end

    module HasQualificationScope
      def items
        params[:has_qualification].present? ? super.with_qualification(params[:has_qualification]) : super
      end
    end

    def EagerLoaded
      def items
        super.includes(:details, :matches, :comments)
      end
    end

    attr_reader :ability, :user, :params

    def initialize(ability, user, params)
      @ability = ability
      @user    = user
      @params  = params

      extend Scope, LeadStatusScope, DepartmentScope, RoleScope, HasQualificationScope, Search
    end

    def items
      # default scope for collection can be applied here
      Prospect.scoped_for_user(user, params[:scope])
    end

    def eager_loaded
      extend EagerLoaded
      self
    end
  end
end