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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