Ruby/SQL code to generate new unique incremented keys (e.g. usernames)

new_unique_incremented_username.rb

# example app code should guarantee a unique, sequenced username:

def calc_prefix(user, fmaxi = 7, lmaxi = 0)
  # e.g.: extract users's first and last name (or use the email example or etc)
  first_name = user.first_name
  last_name = user.last_name
  
  # now, force to lowercase and only (ascii? unicode?) letters and numbers
  # first_name = first_name.downcase.gsub(/[^0-9a-z ]/i, '') # for ascii only
  first_name = first_name.downcase.gsub(/[^\p{Alnum} -]/, '') # for unicode also

  # last_name = last_name.downcase.gsub(/[^0-9a-z ]/i, '') # for ascii only
  last_name = last_name.downcase.gsub(/[^\p{Alnum} -]/, '') # for unicode also

  # and return like 'first.l' ...
  # NOTE: We will limit to [0..fmaxi] chars for first name and likewise for last name
  return "#{first_name[0..(fmaxi)]}.#{last_name[0..(lmaxi)]} # e.g.: "john.s"
end

def highest_count_match_with_or_with_count_suffix(prefix)
  prefix_safer_sql = prefix.gsub('.','\.') # just a '.' means any char, so we must prefix with '\'

  usernames = User.where("username REGEXP /^#{prefix_safer_sql}((\.[0-9]+))*$/gm").pluck(:username) #=> ["john.s", "john.s.0", "john.s.1", "john.s.2", "john.s.3", "john.s.4", "john.s.45", "john.s.456", "john.s.476865"]

  numbers = usernames.map{|un| (un.split('.')[2] || '').to_i} #=> [0, 0, 1, 2, 3, 4, 45, 456, 476865]

  numbers.max
end

def new_username(user, fmaxi = 7, lmaxi = 0)
  prefix = calc_prefix(user, fmaxi, lmaxi)
  highest_count = highest_count_match_with_or_with_count_suffix(prefix)
  "#{prefix}.#{highest_count + 1}
end

# ...

username = new_username(user, fmaxi, lmaxi)

# test variables for the first-name-last-initial prefix matcher would be something like:

all_usernames = %w[
  johnAs
  john.s
  john.s.0
  john.s.1
  john.s.2
  john.s.3
  john.s.4
  john.s.45
  john.s.456
  john.s.476865
  abcjohn.s.4
  abcjohn.s.4xyz
]

expected_matches = %w[
  john.s
  john.s.0
  john.s.1
  john.s.2
  john.s.3
  john.s.4
  john.s.45
  john.s.456
  john.s.476865
]

expected_last_count = 476865
expected_next_count = 476866
expected_username = "john.s.476866"