CustomActiveRecordObserver

A DSL to track create/update/destroy operations based on AR callbacks.

Installation

Add the following code to your Gemfile

# Gemfile
gem 'custom_active_record_observer'

and run bundle install

Then add the observation rules (in config/initializers/events_observation.rb)

# config/initializers/events_observation.rb
CustomActiveRecordObserver.observe 'User' do
  on_create do |user|
    # do something
  end
end

In this expample we track model User. If a new user is created, AR :after_commit callback is launched and the block in on_create executes.

We can track by real classes (which is not always possible, since that code is better to be placed in config/initializers). But passing strings or symbols is also allowed:

  CustomActiveRecordObserver.observe User, :EngagedUser, 'CompanyOwner' { ... }

Here we track three classes.

Usage

How do we use custom_active_record_observer in the wild? For instance, in this example we publish UserRegisteredEvent on user creation. (using rails_event_store gem). This is a pretty useful trade-off if there are no operations or aggregates introduced in the system architecture.

CustomActiveRecordObserver.observe 'User' do
  on_create do |user|
    Rails.application.config.event_store.publish_event(
      UserRegisteredEvent.strict(data: { user_id: user.id }),
      stream_name: "users-#{user.id}"
    )
  end
end

Usage with handler

handler might be passed into the observation. It is a simple object/class which has metho call. The result of do..end block is passed into that handler.

handler = ->(event) do
  Rails.application.config.event_store.publish_event(
    event,
    stream_name: "users-#{ event.data.fetch(:user_id) }"
  )
end

CustomActiveRecordObserver.observe 'User', handler: handler do
  on_create do |user|
    UserRegisteredEvent.strict(data: { user_id: user.id })
  end

  on_add :user_group_id do |user|
    UserAssignedToUserGroup.strict(data: {
      user_id: user.id,
      user_group_id: user.user_group_id
    })
  end

  on_change :user_group_id do |user|
    UserAssignedToAnotherUserGroupEvent.strict(data: {
      user_id: user.id,
      user_group_id: user.user_group_id
    })
  end

  on_remove :user_group_id do |user|
    UserUnassignedFromUserGroupEvent.strict(data: {
      user_id: user.id,
      user_group_id: user.previous_changes.fetch(:user_group_id).first
    })
  end

  on_destroy do |user|
    UserTerminatedEvent.strict(data: { user_id: user.id })
  end
end

Licence

See LICENSE file.

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/OnApp/custom_active_record_observer.