RSpec Query Limit matcher

query_limit.rb

# Allows for assertions regarding number of queries executed.
#
# Usage:
# it "eager loads `#manager` association" do
#   expect do
#     employee = Employee.with_manager.find(employee_id)
#     employee.manager
#   end.to satisfy_query_limit(1)
# end
RSpec::Matchers.define :satisfy_query_limit do |expected|
  match do |block|
    count_queries(&block) <= expected
  end

  failure_message_for_should do |actual|
    "Expected to execute no more than #{expected} SQL queries, got #{@query_count}"
  end

  failure_message_for_should_not do |actual|
    "Expected to execute more than #{expected} SQL queries, got #{@query_count}"
  end

  def count_queries
    @query_count = 0
    filter_list = %w[CACHE SCHEMA]
    # Rails 3.2 introduces ActiveSupport::Notifications::subscribed for temporary
    # subscribers. Use that for 3.2+
    query_counter = ->(name, started, finished, unique_id, payload) do
      @query_count += 1 unless payload[:name].in?(filter_list)
    end
    subscriber = ActiveSupport::Notifications.subscribe("sql.active_record", query_counter)
    yield
    ActiveSupport::Notifications.unsubscribe(subscriber)
    @query_count
  end
end