test in rspec that a block of code does / does not access the DB

access_db_matcher.rb

RSpec::Matchers.define :access_db do
  match do |actual|
    # track db access via ActiveRecord instrumentation
    sql_callback = ->(*, event) do
      # ignore automatic schema queries and transactions, though
      @db_event = event unless event[:sql] =~ / a\.|max_ident|SAVEPOINT/
    end

    ActiveSupport::Notifications.subscribed(sql_callback, 'sql.active_record') do
      actual.call
    end

    @db_event.present?
  end

  failure_message_when_negated do |actual|
    "expected #{actual} not to access the database, " \
      "but it did by running:\n#{@db_event[:name]} - #{@db_event[:sql]}"
  end

  supports_block_expectations
end