Added QueryRecorder to test N+1 fix on Milestone#show

spec/features/milestones/show_spec.rb

+require 'rails_helper'
+
+describe 'Milestone show', feature: true do
+  let(:user) { create(:user) }
+  let(:project) { create(:empty_project) }
+  let(:milestone) { create(:milestone, project: project) }
+  let(:labels) { create_list(:label, 2, project: project) }
+  let(:issue_params) { { project: project, assignee: user, author: user, milestone: milestone, labels: labels } }
+
+  before do
+    project.add_user(user, :developer) 
+    login_as(user)
+  end
+
+  def visit_milestone
+    visit namespace_project_milestone_path(project.namespace, project, milestone)
+  end
+
+  it 'avoids N+1 database queries' do
+    create(:labeled_issue, issue_params)
+    control_count = ActiveRecord::QueryRecorder.new { visit_milestone }.count
+    create_list(:labeled_issue, 10, issue_params)
+
+    expect { visit_milestone }.not_to exceed_query_limit(control_count)
+  end
+end

spec/support/query_recorder.rb

+module ActiveRecord
+  class QueryRecorder
+    attr_reader :log
+
+    def initialize(&block)
+      @log = []
+      ActiveSupport::Notifications.subscribed(method(:callback), 'sql.active_record', &block)
+    end
+
+    def callback(name, start, finish, message_id, values)
+      return if %w(CACHE SCHEMA).include?(values[:name])
+      @log << values[:sql]
+    end
+
+    def count
+      @log.count
+    end
+
+    def log_message
+      @log.join("\n\n")
+    end
+  end
+end
+
+RSpec::Matchers.define :exceed_query_limit do |expected|
+  supports_block_expectations
+
+  match do |block|
+    query_count(&block) > expected
+  end
+
+  failure_message_when_negated do |actual|
+    "Expected a maximum of #{expected} queries, got #{@recorder.count}:\n\n#{@recorder.log_message}"
+  end
+
+  def query_count(&block)
+    @recorder = ActiveRecord::QueryRecorder.new(&block)
+    @recorder.count
+  end
+end