When a complex filtering solution is required to be built the code becomes a mess more often than not. Filtering records is a pretty simple task on it’s own: just apply required ActiveRecord scopes on a certain conditions of the given params. Those conditions, however, can become much more complex than you expect. As a result, the code spreads out in controller, models and logic layers making it hardly readable and maintainable. Further down, I’ll describe a simple approach that’ll help avoid such annoyances.
First off, we want to delegate all the filtering, ordering, pagination and other collection presentation stuff to a separate class so that controller code became as simple as possible (example code uses CanCan gem, which is augmenting collection pattern usage):
class LeadsController < ApplicationController def index # current_ability – CanCan method collection = LeadsCollection.new(current_ability, current_user, params) @leads = collection.eager_loaded.paginated.items end end
I assume that this is not a single occurrence of collection functionality in the system. So, common behavior can be extracted to some base class:
class BaseCollection attr_reader :ability, :user, :params module Authorization def items # CanCan stuff super.accessible_by(ability) end end module Ordering def items if order_key super.order(ordering_arel) else super end end private def ordering_arel self.arel_table(order_key.to_sym).send(order_dir) end def order_key params[:order_key] end def order_dir params[:order_dir] =~ /\Adesc\z/i ? :desc : :asc end end module Pagination def items super.page(params[:page]).per(params[:per]) end end module Search def items # Assuming search_for is implemented for model super.search_for(params[:search]) end end end
Here comes Criteria Pattern in play. The idea is to encapsulate collection aspects into small manageable pieces of code and chain them by means of extending collection object with those modules:
class LeadsCollection < BaseCollection def initialize(ability, user, params) @ability = ability @user = user @params = params extend Authorization, Ordering, Pagination, Search end end
Certainly, my example above is oversimplified and does not differ from the base class. Let’s add some functionality to make this example have a bit more sense:
class LeadsCollection < BaseCollection module LeadsStatusScope def items # Here and below some scopes are applied. # We assume they all properly implemented in model super.in_status(params[:lead_status]) end end module DepartmentScope def items if department_id_given? super.with_department(department_id) else super end end private def department_id params[:department] end def department_id_given? department_id && Department.where(id: department_id).exists? end end module RoleScope def items params[:role].present? ? super.with_role(params[:role]) : super end end module HasQualificationScope def items if params[:has_qualification].present? super.with_qualification(params[:has_qualification]) else super end end end def initialize(ability, user, params) @ability = ability @user = user @params = params extend Authorization, Ordering, Search, LeadStatusScope, DepartmentScope, RoleScope, HasQualificationScope end def items # default scope for collection can be applied here Lead.scoped_for_user(user, params[:scope]) end end
For the sake of making the code reusable (e. g. for reports, exports etc.) some modules can be applied by calling separate method. You can see it in the first code insert for controller: methods #paginated and #eager_loaded. Let’s add them to our collection:
class LeadsCollection < BaseCollection ... def EagerLoaded def items super.includes(:details, :matches, :comments) end end ... def eager_loaded extend EagerLoaded self end def paginated # Pagination module is defined in BaseCollection class extend Pagination self end end
Sure thing, you have to be accurate with chaining #items methods. Some things will not work together, e.g. using include for eager loading in one module and defining custom select fields in another and then chaining them together.
So, this is it. I hope this collection approach will come in handy in your work.