Adding text about conditional callbacks

1 files changed, 51 insertions, 1 deletions

diff --git a/railties/doc/guides/source/activerecord_validations_callbacks.txt b/railties/doc/guides/source/activerecord_validations_callbacks.txt
index 7d37df1ed2..09cc1fdb20 100644
--- a/railties/doc/guides/source/activerecord_validations_callbacks.txt
+++ b/railties/doc/guides/source/activerecord_validations_callbacks.txt
@@ -359,7 +359,7 @@ Sometimes it will make sense to validate an object just when a given predicate i
 
 === Using a symbol with the +:if+ and +:unless+ options
 
-You can associated the +:if+ and +:unless+ options with a symbol corresponding to the name of a method that will get called right before validation happens. This is the most commonly used option.
+You can associate the +:if+ and +:unless+ options with a symbol corresponding to the name of a method that will get called right before validation happens. This is the most commonly used option.
 
 [source, ruby]
 ------------------------------------------------------------------
@@ -678,6 +678,56 @@ In Rails, the preferred way of registering callbacks is by using macro-style cla
 
 CAUTION: Remember to always declare the callback methods as being protected or private. These methods should never be public, otherwise it will be possible to call them from code outside the model, violating object encapsulation and exposing implementation details.
 
+== Conditional callbacks
+
+Like in validations, we can also make our callbacks conditional, calling then only when a given predicate is satisfied. You can do that by using the +:if+ and +:unless+ options, which can take a symbol, a string or a Ruby Proc. You may use the +:if+ option when you want to specify when the callback *should* get called. If you want to specify when the callback *should not* be called, then you may use the +:unless+ option.
+
+=== Using a symbol with the +:if+ and +:unless+ options
+
+You can associate the +:if+ and +:unless+ options with a symbol corresponding to the name of a method that will get called right before the callback. If this method returns +false+ the callback won't be executed. This is the most common option. Using this form of registration it's also possible to register several different methods that should be called to check the if the callback should be executed.
+
+[source, ruby]
+------------------------------------------------------------------
+class Order < ActiveRecord::Base
+  before_save :normalize_card_number, :if => :paid_with_card?
+end
+------------------------------------------------------------------
+
+=== Using a string with the +:if+ and +:unless+ options
+
+You can also use a string that will be evaluated using +:eval+ and needs to contain valid Ruby code. You should use this option only when the string represents a really short condition.
+
+[source, ruby]
+------------------------------------------------------------------
+class Order < ActiveRecord::Base
+  before_save :normalize_card_number, :if => "paid_with_card?"
+end
+------------------------------------------------------------------
+
+=== Using a Proc object with the +:if+ and :+unless+ options
+
+Finally, it's possible to associate +:if+ and +:unless+ with a Ruby Proc object. This option is best suited when writing short validation methods, usually one-liners.
+
+[source, ruby]
+------------------------------------------------------------------
+class Order < ActiveRecord::Base
+  before_save :normalize_card_number, 
+    :if => Proc.new { |order| order.paid_with_card? }
+end
+------------------------------------------------------------------
+
+=== Multiple Conditions for Callbacks
+
+When writing conditional callbacks, it's possible to mix both +:if+ and +:unless+ in the same callback declaration.
+
+[source, ruby]
+------------------------------------------------------------------
+class Comment < ActiveRecord::Base
+  after_create :send_email_to_author, :if => :author_wants_emails?, 
+    :unless => Proc.new { |comment| comment.post.ignore_comments? }
+end
+------------------------------------------------------------------
+
 == Available callbacks
  
 Here is a list with all the available Active Record callbacks, listed in the same order in which they will get called during the respective operations.