ActiveModel::Observer
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 < 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/application.rb file.
config.active_record.observers = :comment_observer, :signup_observer
Observers will not be invoked unless you define these in your application configuration.
Observers register themselves in the model class they observe, since it is the class that notifies them of events when they occur. As a side-effect, when an observer is loaded its corresponding model class is loaded.
Up to (and including) Rails 2.0.2 observers were instantiated between plugins and application initializers. Now observers are loaded after application initializers, so observed models can make use of extensions.
If by any chance you are using observed models in the initialization you can still load their observers by calling ModelObserver.instance before. Observers are singletons and that call instantiates and registers them.
# File lib/active_record/observer.rb, line 100 100: def self.method_added(method) 101: method = method.to_sym 102: 103: if ActiveRecord::Callbacks::CALLBACKS.include?(method) 104: self.observed_methods += [method] 105: self.observed_methods.freeze 106: end 107: end
# File lib/active_record/observer.rb, line 119 119: def add_observer!(klass) 120: super 121: define_callbacks klass if observe_callbacks? 122: end
# File lib/active_record/observer.rb, line 124 124: def define_callbacks(klass) 125: existing_methods = klass.instance_methods.map { |m| m.to_sym } 126: observer = self 127: observer_name = observer.class.name.underscore.gsub('/', '__') 128: 129: self.class.observed_methods.each do |method| 130: callback = :"_notify_#{observer_name}_for_#{method}" 131: unless existing_methods.include? callback 132: klass.send(:define_method, callback) do # def _notify_user_observer_for_before_save 133: observer.update(method, self) # observer.update(:before_save, self) 134: end # end 135: klass.send(method, callback) # before_save :_notify_user_observer_for_before_save 136: end 137: end 138: end
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