Observers in Rails
##Life without Observers
Let’s say you need to listen to changes in a model’s attributes and trigger some events based on those changes. If you look at the following example, I’ve done exaclty that. If the status of application is changed an email is sent.
The above piece of code violates the Single Responsibility Principle, which states that each class should have “only one responsiblilty”. The Application
model should only deal with saving and manupluating the application object. Sending emails is not part of it’s responsiblity.
##ActiveRecord::Observers
We can delegate the responsiblity of observing changes in model attibutes and responding to those changes, to a seperate Observer class. Model specific event handling is much cleaner this way. This way, we can refrain from polluting the model by adding not un-necessary methods.
You can read more about the observer design pattern here.
##Note
- Observer names are infered form the model names. So if you name your observer as
ApplicationObserver
, rails knows that it’s observing theApplication
model. - Your observers need not be model specific. SRP states that “Responsibility should be entirely encapsulated by the context”. So by this definition you can have a single observer which listens to changes in multiple models and perfom a single function say, sending an email or publishing a notification. To acomplish this you can use the
ActiveRecord::Observer.observe
method. - Place your observers in
app/models
orapp/models/observers
. - Finally, Observers have been removed form the rails core after
Rails 3.2
. You need to include therails-observers
gem to use observers in later rails versions.