Custom Rspec Matcher for N+1 Queries in Rails

Rails and Active Record make many things easy: connecting to the database, building complex queries based on your object model, and easily migrating your schema up, down, and sideways. They also let you very easily introduce N+1 queries.

Using N+1 Queries

N+1 queries are common when working with an ORM like Active Record. Active Record lazily loads relationships when querying. The developer has to explicitly ask for things to preload via includes.

For example, you can trigger one query for the car and N more to get the driver of each car.


  cars = Car.where(color: :red)
  cars.each do |car|
    puts car.driver.first_name
  end

Preloading the Relationship

Calling includes preloads the relationship. By adding the includes, Rails will either do the needed JOIN or a separate query/queries (for multiple relationships).

  cars = Car.where(color: :red).includes(:driver)
  cars.each do |car|
    puts car.driver.first_name
  end

Finding N+1 Queries

A great way to see where N+1 queries have sneaked into your Rails app is to use the bullet. New Relic, Skylight, and others can help you find problematic queries.

Once I find a problem query, I like to write a preloading rspec to make sure I’ve properly preloaded what I need. I’ll often wrap groups of common includes into named scopes to keep things a little cleaner. I’ve also added a matcher, not_talk_to_db, that mocks out the database to make sure no calls are made to load additional data.

  it 'preloads drivers' do
    cars = Car.with_driver.all
    expect(->() {
      # ask for all the things that should have 
      # been preloaded
      cars.map{|c|c.driver.first_name}
    }).to not_talk_to_db
  end
  RSpec::Matchers.define :not_talk_to_db do |_expected|
    match do |block_to_test|
      %w(exec_delete exec_insert exec_query exec_update).each do |meth|
        expect(ActiveRecord::Base.connection).not_to receive(meth)
      end
      block_to_test.call
    end
    supports_block_expectations
  end

N+1 queries are quick to sneak in and can wreak havoc on performance. I hope this approach will help you find them quickly, write the test, and preload correctly. Good luck on your Active Record adventures.