discourse/app/models/invite_redeemer.rb

InviteRedeemer = Struct.new(:invite, :username, :name, :password, :user_custom_fields) do

  def redeem
    Invite.transaction do
      if invite_was_redeemed?
        process_invitation
        return invited_user
      end
    end

    nil
  end

  # extracted from User cause it is very specific to invites
  def self.create_user_from_invite(invite, username, name, password = nil, user_custom_fields = nil)
    if username && UsernameValidator.new(username).valid_format? && User.username_available?(username)
      available_username = username
    else
      available_username = UserNameSuggester.suggest(invite.email)
    end
    available_name = name || available_username

    user_params = {
      email: invite.email,
      username: available_username,
      name: available_name,
      active: false,
      trust_level: SiteSetting.default_invitee_trust_level
    }

    user = User.unstage(user_params)
    user = User.new(user_params) if user.nil?

    if !SiteSetting.must_approve_users? || (SiteSetting.must_approve_users? && invite.invited_by.staff?)
      ReviewableUser.set_approved_fields!(user, invite.invited_by)
    end

    user_fields = UserField.all
    if user_custom_fields.present? && user_fields.present?
      field_params = user_custom_fields || {}
      fields = user.custom_fields

      user_fields.each do |f|
        field_val = field_params[f.id.to_s]
        fields["#{User::USER_FIELD_PREFIX}#{f.id}"] = field_val[0...UserField.max_length] unless field_val.blank?
      end
      user.custom_fields = fields
    end

    user.moderator = true if invite.moderator? && invite.invited_by.staff?

    if password
      user.password = password
      user.password_required!
    end

    user.save!

    if invite.via_email
      user.email_tokens.create!(email: user.email)
      user.activate
    end

    User.find(user.id)
  end

  private

  def invited_user
    @invited_user ||= get_invited_user
  end

  def process_invitation
    approve_account_if_needed
    add_to_private_topics_if_invited
    add_user_to_groups
    send_welcome_message
    notify_invitee
    delete_duplicate_invites
  end

  def invite_was_redeemed?
    # Return true if a row was updated
    mark_invite_redeemed == 1
  end

  def mark_invite_redeemed
    Invite.where('id = ? AND redeemed_at IS NULL AND created_at >= ?',
                 invite.id, SiteSetting.invite_expiry_days.days.ago)
      .update_all('redeemed_at = CURRENT_TIMESTAMP')
  end

  def get_invited_user
    result = get_existing_user
    result ||= InviteRedeemer.create_user_from_invite(invite, username, name, password, user_custom_fields)
    result.send_welcome_message = false
    result
  end

  def get_existing_user
    User.where(admin: false, staged: false).find_by_email(invite.email)
  end

  def add_to_private_topics_if_invited
    topic_ids = Topic.where(archetype: Archetype::private_message).includes(:invites).where(invites: { email: invite.email }).pluck(:id)
    topic_ids.each do |id|
      TopicAllowedUser.create(user_id: invited_user.id, topic_id: id) unless TopicAllowedUser.exists?(user_id: invited_user.id, topic_id: id)
    end
  end

  def add_user_to_groups
    new_group_ids = invite.groups.pluck(:id) - invited_user.group_users.pluck(:group_id)
    new_group_ids.each do |id|
      invited_user.group_users.create(group_id: id)
    end
  end

  def send_welcome_message
    if Invite.where('email = ?', invite.email).update_all(['user_id = ?', invited_user.id]) == 1
      invited_user.send_welcome_message = true
    end
  end

  def approve_account_if_needed
    if invited_user.present? && reviewable_user = ReviewableUser.find_by(target: invited_user)
      reviewable_user.perform(
        invite.invited_by,
        :approve,
        send_email: false,
        approved_by_invite: true
      )
    end
  end

  def notify_invitee
    if inviter = invite.invited_by
      inviter.notifications.create(notification_type: Notification.types[:invitee_accepted],
                                   data: { display_username: invited_user.username }.to_json)
    end
  end

  def delete_duplicate_invites
    Invite.where('invites.email = ? AND redeemed_at IS NULL AND invites.id != ?', invite.email, invite.id).delete_all
  end
end