query_tracer.rb

class QueryTracer
  def disallow_queries
    return yield unless Rails.env.test?

    ActiveSupport::Notifications.unsubscribe("sql.active_record")

    subscriber = ActiveSupport::Notifications.subscribe(
      "sql.active_record"
    ) do |_name, _start, _finish, _id, payload|
      if payload[:sql].match?(/select/i) && !payload[:sql].include?("pg_")
        raise <<-EOS.strip_heredoc
          Queries are disallowed currently by QueryTracer#disallow_queries"
          Query:
            #{payload[:sql]}
        EOS
      end
    end

    result = yield

    ActiveSupport::Notifications.unsubscribe(subscriber)

    result
  end
end

# -----------------------

users = User.some.set.of.scopes.includes(...).load

QueryTracer.new.disallow_queries do
  users.each do |user|
    some_method(user)
  end
end