join users with their latest device by created_at

gistfile1.txt

SELECT
	u.*,
	row_to_json(d.*) device
FROM
	users u
	INNER JOIN (
		SELECT
			user_id,
			MAX(created_at) MaxDate
		FROM
			devices
		GROUP BY
			user_id) MaxDates ON u.id = MaxDates.user_id
	LEFT OUTER JOIN devices d ON MaxDates.user_id = d.user_id
		AND MaxDates.MaxDate = d.created_at