Class | ActiveRecord::Observer |
In: |
vendor/rails/activerecord/lib/active_record/observer.rb
|
Parent: | Object |
Observer classes respond to lifecycle 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 < ActiveRecord::Observer def after_save(comment) Notifications.deliver_comment("admin@do.com", "New comment was posted", comment) end end
This Observer sends an email when a Comment#save is finished.
class ContactObserver < ActiveRecord::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.
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 < ActiveRecord::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 < ActiveRecord::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.
The observer can implement callback methods for each of the methods described in the Callbacks module.
If you‘re using Active Record within Rails, observer classes are usually stored in app/models with the naming convention of app/models/audit_observer.rb.
In order to activate an observer, list it in the config.active_record.observers configuration setting in your config/environment.rb file.
config.active_record.observers = :comment_observer, :signup_observer
Observers will not be invoked unless you define these in your application configuration.
Start observing the declared classes and their subclasses.
# File vendor/rails/activerecord/lib/active_record/observer.rb, line 151 151: def initialize 152: Set.new(observed_classes + observed_subclasses).each { |klass| add_observer! klass } 153: end
Attaches the observer to the supplied model classes.
# File vendor/rails/activerecord/lib/active_record/observer.rb, line 133 133: def observe(*models) 134: models.flatten! 135: models.collect! { |model| model.is_a?(Symbol) ? model.to_s.camelize.constantize : model } 136: define_method(:observed_classes) { Set.new(models) } 137: end
The class observed by default is inferred from the observer‘s class name:
assert_equal [Person], PersonObserver.observed_class
# File vendor/rails/activerecord/lib/active_record/observer.rb, line 141 141: def observed_class 142: if observed_class_name = name.scan(/(.*)Observer/)[0] 143: observed_class_name[0].constantize 144: else 145: nil 146: end 147: end
# File vendor/rails/activerecord/lib/active_record/observer.rb, line 176 176: def add_observer!(klass) 177: klass.add_observer(self) 178: klass.class_eval 'def after_find() end' unless klass.respond_to?(:after_find) 179: end
# File vendor/rails/activerecord/lib/active_record/observer.rb, line 168 168: def observed_classes 169: Set.new([self.class.observed_class].compact.flatten) 170: end