A custom rspec matcher to catch n+1 queries

not_execute_n_plus_one_matcher.rb

# use this matched to catch most n+1 queries in request specs by creating at least three records, then 
# doing something like expect { subject }.to not_execute_n_plus_one

RSpec::Matchers.define :not_execute_n_plus_one do
  match do |block|
    @queries = []
    ActiveSupport::Notifications.subscribe 'sql.active_record' do |_, _, _, _, values|
      next if %w(CACHE SCHEMA).include? values[:name]
      next if values[:sql].start_with?("SAVEPOINT")
      next if values[:sql].start_with?("RELEASE SAVEPOINT")
      next if values[:sql].start_with?("INSERT INTO")

      @queries << values[:sql]
    end
    block.call
    @sqls = @queries.reduce({}) do |acc, q|
      acc["\"#{q}\""] ||= 0
      acc["\"#{q}\""] += 1
      acc
    end.select { |_k, v| v > 2 }
    @sqls.empty?
  end

  supports_block_expectations

  failure_message do |_actual|
    sql_messages = @sqls.map do |k, v|
      "#{k} (executed #{v} times)"
    end
    "The following n+1(s) queries were found: \n#{sql_messages.join("\n")}"
  end
end