Query Objects
They store complex SQL queries, data aggregation, and filtering methods.
The problem
Say you have the following view.
class PostsController < ApplicationController def index @posts = Post .joins('LEFT OUTER JOIN users ON users.id = posts.author_id') .where(published: true) .where('view_count > ?', params[:min_view_count]) .where('users.first_name LIKE ?', "#{params[:author_name]}%") end It's a lot of code for a controller, so let's refactor it to use a Query Object.
The PostsQuery object
class PostsQuery def self.call(params = {}, relation = Post.all) relation = published(relation) relation = minimal_view_count(relation, params[:view_count]) relation = author_first_name_like(relation, params[:first_name]) relation end def self.published(relation) relation.where(published: true) end def self.minimal_view_count(relation, view_count) return relation unless view_count.present? relation.where('view_count > ?', view_count) end def self.author_first_name_like(relation, first_name) return relation unless first_name.present? with_authors(relation) .where('users.first_name LIKE ?', "#{first_name}%") end def self.with_authors(relation) relation.joins('LEFT OUTER JOIN users ON users.id = posts.author_id') end end The implementation
Nice, let's see how the controller looks now.
class PostsController < ApplicationController def index @posts = PostsQuery.call(params) end end Final thoughts
Not only looks better, testing this approach is much simpler.
This does not only applies for Rails apps, this pattern could be used anywhere in Ruby.
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