Useful queries
Find duplicated username
User.find_by_sql(' SELECT username, COUNT(id) FROM users GROUP BY username HAVING COUNT(id) > 1 ORDER BY COUNT(id) ').pluck(:username)
Find users who has no profile
User.includes(:profile).where(profile: { user_id: nil }) # OR User.left_outer_joins(:profile).where(profile: { id: nil })
Query the latest locations of each User (user has_many :locations)
Location.where( id: Location.select('MAX(id) AS max_id').group(:user_id) )