mastodon/app/services/setup_local_account_service.rb

class SetupLocalAccountService < BaseService
  # Setup an account for a new user instance by generating
  # an RSA key pair and a profile
  # @param [User] user Unsaved user instance
  # @param [String] username
  def call(user, username)
    user.build_account

    user.account.username = username
    user.account.domain   = nil

    user.save!
  end
end