Refactoring towards query objects in Rails

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.