Friday, July 29, 2011

Difference between Validations, Callbacks and Observers

Validations allow you to ensure that only valid data is stored in your database.
Example: validates_presence_of :user_name, :password
                 validates_numericality_of :value

We can write custom validation also as

def validate
  errors.add(:price, “should be a positive value”) if price.nil?|| price < 0.01

Callbacks and observers allow you to trigger logic before or after an alteration of an object’s state.

Callbacks are methods that get called at certain moments of an object’s life cycle. With callbacks it’s possible to write code that will run whenever an Active Record object is created, saved, updated, deleted, validated, or loaded from the database.

Callbacks are hooks into the life cycle of an Active Record object that allow you to trigger logic before or after an alteration of the object state. This can be used to make sure that associated and dependent objects are deleted when destroy is called (by overwriting before_destroy) or to massage attributes before they’re validated (by overwriting before_validation)

Observers are similar to callbacks, but with important differences. Whereas callbacks can pollute a model with code that isn’t directly related to its purpose, observers allow you to add the same functionality outside of a model. For example, it could be argued that a User model should not include code to send registration confirmation emails. Whenever you use callbacks with code that isn’t directly related to your model, you may want to consider creating an observer instead.

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