Database queries are common when you develop web applications. Ruby on Rails and it’s ActiveRecord liberates you from writing tons of boilerplate SQL code and results in creation of elegant, eloquent queries in plain Ruby.

But plenty of immense possibilities that Ruby and ActiveRecord provide, unfortunately, remain unused. I bet that often you see a lot of enormous scopes in Ruby on Rails models, endless chains of queries in controllers and even bulky chunks of raw SQL.

#+begin_src ruby @articles = Article.includes(:user) .order(“created_at DESC”) .where(“text IS NOT NULL”) .page(page)

@articles = Articles.connection .select_all(%Q{SELECT articles.* FROM articles WHERE (text IS NOT NULL) ORDER BY created_at DESC LIMIT 5 OFFSET 0}) #+end_src Bad cases of using ActiveRecord queries

These poor practices may create obstacles and become a reason of developer’s headaches in the real-world web applications.

Typical DB queries application problems:

• Big pieces of queries code in controllers/models/services mess up your code
• It is hard to understand complex database requests
• Inserts of raw SQL are non-consistent and often mix with ActiveRecord queries
• Testing one separate query in isolation is very problematic
• It is difficult to compose, extend or inherit queries
• Often Single Responsibility Principle gets violated

Solution:

These problems can be solved by using Query Object pattern — a common technique that isolates your complex queries.

Query Object in ideal case is a separate class that contains one specific query that implements just one business logic rule.

Implementation:

For most of the cases Query Object is PORO that accepts relation in constructor and defines queries named like an ActiveRecord common methods:

#+begin_src ruby

class Article < ActiveRecord::Base scope :by_title, ->(direction) { order title: direction } scope :by_date, ->(direction) { order created_at: direction } scope :by_author, ->(direction) { order “users.full_name #{direction}” }

SORT_OPTIONS = %w(by_date by_title by_author).freeze

 1  def initialize(params = {}, relation = Article.includes(:user))
 2    @relation = relation
 3    @params = params
 4  end
 5
 6  def all
 7    @relation.public_send(sort_by, direction)
 8  end
 9
10  private
11
12  def sort_by
13    @params[:sort].presence_in(SORT_OPTIONS) || :by_date
14  end
15
16  def direction
17    @params[:direction] == "asc" ? :asc : :desc
18  end

end

index @articles = OrderedArticlesQuery.new(sort_query_params).all.page(params[:page]) end

1  private
2
3  def sort_query_params
4    params.slice(:sort_by, :direction)
5  end

end #+end_src Query Object implementation and usage in controller

HEREDOC syntax for raw SQL:

For the cases where you desperately need to use raw SQL code try to isolate it using Ruby’s HEREDOC syntax:

 1 class PopularArticlesQuery
 2  POPULAR_TRESHOLD = 5
 3
 4  def initialize(subscriptions = Subscription.all)
 5    @subscriptions = subscriptions
 6  end
 7
 8  def all
 9    @subscriptions.where(query)
10  end
11
12  private
13
14  def query
15    <<-SQL
16      articles.comments_count >= #{POPULAR_TRESHOLD}
17      AND articles.content IS NOT NULL
18      AND articles.status = #{Article::STATUSES[:published]}
19      ORDER BY articles.comments_count DESC
20    SQL
21  end
22end

HEREDOC syntax example for raw SQL inserts

Extending scope:

If your scope relates to existing QueryObject, you can easily extend its relation instead of cluttering up your models. ActiveRecord::QueryMethods.extending method will help you:

 1class OrderedArticlesQuery
 2  SORT_OPTIONS = %w(by_date by_title by_author).freeze
 3
 4  def initialize(params = {}, relation = Article.includes(:user))
 5    @relation = relation.extending(Scopes)
 6
 7    @params = params
 8  end
 9
10  def all
11    @relation.public_send(sort_by, direction)
12  end
13
14  private
15
16  def sort_by
17    @params[:sort].presence_in(SORT_OPTIONS) || :by_date
18  end
19
20  def direction
21    @params[:direction] == "asc" ? :asc : :desc
22  end
23
24  # Group additional scope methods in module in order to extend relation
25  module Scopes
26    def by_title(direction)
27      order(title: direction)
28    end
29
30    def by_date(direction)
31      order(created_at: direction)
32    end
33
34    def by_author
35      order("users.full_name #{direction}")
36    end
37  end
38end

Extending scope for Query Objects relations

Composing Query Objects:

Query Objects should be devised to support composition with other Query Objects and other ActiveRecord relations. In the example below two composed Query Objects represent one SQL query:

 1class FeaturedQuery
 2  def initialize(relation = Article.all)
 3    @relation = relation
 4  end
 5
 6  def all
 7    @relation.where(featured: true).where("views_count > ?", 100)
 8  end
 9end
10
11class ArticlesController
12  def index
13    @articles = FeaturedArticlesQuery.new(sorted_articles).all # SELECT "articles".* FROM "articles" WHERE "articles"."featured" = $1 # AND (views_count >100) ORDER BY "articles"."created_at" DESC LIMIT 10 OFFSET 0
14  end
15
16  private
17
18  def sorted_articles
19    SortedArticlesQuery.new(sort_query_params).all
20  end
21
22  def sort_query_params
23    { sort: :by_title, direction: :desc }
24  end
25end

Composing two Query Objects

Inheritance of Query Objects:

If you have similar queries you may want them to be inherited to reduce repetition and follow DRY principle:

 1class ArticlesQuery
 2  TEXT_LENGTH = 3
 3
 4#+BEGIN_EXAMPLE
 5  def initialize(comments = Comment.all)
 6    @comments = comments
 7  end
 8
 9  def all
10    comments
11      .where("user_id IS NOT NULL")
12      .where("LENGTH(content) #{condition}")
13  end
14
15  def condition
16    "> #{TEXT_LENGTH}"
17  end
18end
19
20class LongArticlesQuery < ArticlesQuery
21  TEXT_LENGTH = 5
22
23  #+BEGIN_EXAMPLE
24  def condition
25    ">= #{TEXT_LENGTH}"
26  end
27end

Inheritance of Query Objects

Testing Query Objects:

Query Objects should be designed to be pleasant for testing. In most cases you just need to test core methods defined in query for their results:

 1require "rails_helper"
 2
 3describe LongArticlesQuery do
 4  describe "#all" do
 5    subject(:all) { described_class.new.all }
 6
 7    before do
 8      create :article, text: "abc"
 9      create :article, text: "this is long article"
10    end
11
12    it "returns one short comment" do
13      expect(all.size).to eq(1)
14    end
15  end
16end

Testing Query Objects

Summary:

Good Query Object:

• Follows Single Responsibility Principle
• Can be easily tested in isolation
• Can be combined/extended with another Query Object
• Can be effortlessly reused in any other parts of an application
• Returns ActiveRecord::Relation, not Array
• Represents only database query, not business logic or action
• Methods of Query Object are named like ActiveRecord methods (all, last, count, etc)

Use Query Objects when:

• You need to reuse one query in multiple places of application
• You need to extend, compose or inherit queries and their relations
• You need to write a lot of raw SQL, but don’t want to mess up your code
• Your query is too complex / vast for just one method or scope
• Your query causes feature envy

Don’t use Query Objects when:

• Your query is simple enough for just one method or scope
• You don’t need to extend, compose or inherit your query
• Your query is unique and you don’t want to make it reusable

I hope this article will help you to build awesome queries in your applications. Good luck and happy coding!