Active Model Observers
Observer classes respond to life cycle callbacks to implement trigger-like behavior outside the original class. This is a great way to reduce the clutter that normally comes when the model class is burdened with functionality that doesn't pertain to the core responsibility of the class. Example:
class CommentObserver < ActiveModel::Observer def after_save(comment) Notifications.comment("admin@do.com", "New comment was posted", comment).deliver end end
This Observer sends an email when a Comment#save is finished.
class ContactObserver < ActiveModel::Observer def after_create(contact) contact.logger.info('New contact added!') end def after_destroy(contact) contact.logger.warn("Contact with an id of #{contact.id} was destroyed!") end end
This Observer uses logger to log when specific callbacks are triggered.
Observing a class that can't be inferred
Observers will by default be mapped to the class with which they share a
name. So CommentObserver will be tied to observing Comment,
ProductManagerObserver to ProductManager, and so on. If you want to name
your observer differently than the class you’re interested in observing,
you can use the Observer.observe
class method which takes
either the concrete class (Product) or a symbol for that class (:product):
class AuditObserver < ActiveModel::Observer observe :account def after_update(account) AuditTrail.new(account, "UPDATED") end end
If the audit observer needs to watch more than one kind of object, this can be specified with multiple arguments:
class AuditObserver < ActiveModel::Observer observe :account, :balance def after_update(record) AuditTrail.new(record, "UPDATED") end end
The AuditObserver will now act on both updates to Account and Balance by treating them both as records.
If you’re using an Observer in a Rails application with Active Record, be sure to read about the necessary configuration in the documentation for ActiveRecord::Observer.
- Singleton
Start observing the declared classes and their subclasses.
Attaches the observer to the supplied model classes.
The class observed by default is inferred from the observer’s class name:
assert_equal Person, PersonObserver.observed_class
Returns an array of Classes to observe.
You can override this instead of using the observe
helper.
class AuditObserver < ActiveModel::Observer def self.observed_classes [Account, Balance] end end