如何检测Mongoid的HABTM关系变更并在添加对象时发送邮件？

Hey there! Let's tackle this problem step by step—we'll set up logic to detect when a User is added to an Account's managers HABTM association and trigger an email notification.

1. Track Changes to the HABTM Association #

Mongoid's HABTM associations rely on the underlying _ids field (in this case, manager_ids on the Account model) to track related records. We can use Mongoid's built-in change tracking methods to compare the state of this field before and after a save.

Add these callbacks and helper methods to your Account model:

class Account
  include Mongoid::Document
  has_and_belongs_to_many :managers, class_name: 'User', inverse_of: :accounts

  # Store the previous state of manager_ids before saving
  before_save :store_previous_manager_ids

  # Trigger email logic only if manager_ids has changed
  after_save :send_manager_added_emails, if: -> { manager_ids_changed? }

  private

  def store_previous_manager_ids
    # Save a copy of the original manager_ids to compare later
    @previous_manager_ids = manager_ids_was.dup
  end

  def send_manager_added_emails
    # Calculate which manager IDs were newly added
    new_manager_ids = manager_ids - @previous_manager_ids
    return if new_manager_ids.empty?

    # Fetch the corresponding User records
    new_managers = User.find(new_manager_ids)

    # Send a notification email to each newly added manager
    new_managers.each do |manager|
      AccountMailer.manager_added_notification(self, manager).deliver_now
    end
  end
end

Key Details: #

• manager_ids_was: Mongoid provides this method to access the value of manager_ids before any changes were made. We duplicate it to avoid reference issues.
• manager_ids_changed?: A built-in predicate method that returns true if the manager_ids field has been modified since the last save.
• We calculate new_manager_ids by subtracting the previous IDs from the updated IDs—this gives us exactly the users added to the association.

2. Set Up the Email Notification #

Next, create a mailer to handle the notification. Here's an example of what AccountMailer might look like:

class AccountMailer < ApplicationMailer
  def manager_added_notification(account, manager)
    @account = account
    @manager = manager

    mail(
      to: manager.email,
      subject: "You've been added as a manager for #{account.name}"
    )
  end
end

Don't forget to create the corresponding email template (e.g., app/views/account_mailer/manager_added_notification.html.erb) with the content you want to send to the new manager.

3. Test the Workflow #

Using your console example, here's how this would work:

[15] pry(main)> a.reload => #<Account _id: 5ae71215a8abbc6378b4bee8, code: "ACC5", name: "Account5", manager_ids: []>
[16] pry(main)> u.reload => #<User _id: 5ae59b5da8abbc6f3d880a10, name: "Manager 1", account_ids: []>
[17] pry(main)> a.managers << u
[18] pry(main)> a.save

After running a.save, the send_manager_added_emails method will detect that manager_ids changed from an empty array to include u.id, fetch the User record, and send the notification email.

Bonus: Handling Bulk Additions #

This logic works even if you add multiple managers at once (e.g., a.managers << u1 << u2)—it will identify all newly added users and send emails to each one.

内容的提问来源于stack exchange，提问作者tomatito