Tests N+1 queries in Rails using RSpec

test_n_plus_ones_spec.rb

# frozen_string_literal: true

RSpec.describe TestNPlusOnes do
  describe "<something>" do
    subject(:generate) { puts("<Do something with the DB>") }
    # Technique seen in ActiveRecord's #assert_queries_count to use with Minitest:
    # https://github.com/rails/rails/blob/0496a5f994f134695bf7bdc02dacdf24925bc67c/activerecord/lib/active_record/testing/query_assertions.rb#L99
    it "avoids N+1 queries" do
      number_of_queries = 0
      counter = ->(*, payload) {
        return if payload[:cached]

        if payload[:name] != "SCHEMA"
          # puts payload[:sql] # This shows the actual query
          number_of_queries += 1
        end
      }

      ActiveRecord::Base.connection.materialize_transactions
      ActiveSupport::Notifications.subscribed(counter, "sql.active_record") do
        generate
        expect(number_of_queries).to eq(2)
      end
    end
  end
end