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document the instance_reader option for class_attribute

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1 parent d0c4fd7 commit 427b07baa16ac4f61e0762e52e716fe9937b498a @vijaydev vijaydev committed Jun 19, 2011
Showing with 15 additions and 3 deletions.
  1. +15 −3 railties/guides/source/active_support_core_extensions.textile
@@ -947,7 +947,7 @@ h4. Class Attributes
h5. +class_attribute+
-The method +class_attribute+ declares one or more inheritable class attributes that can be overridden at any level down the hierarchy:
+The method +class_attribute+ declares one or more inheritable class attributes that can be overridden at any level down the hierarchy.
<ruby>
class A
@@ -983,7 +983,7 @@ self.default_params = {
}.freeze
</ruby>
-They can be also accessed and overridden at the instance level:
+They can be also accessed and overridden at the instance level.
<ruby>
A.x = 1
@@ -996,7 +996,7 @@ a1.x # => 1, comes from A
a2.x # => 2, overridden in a2
</ruby>
-The generation of the writer instance method can be prevented by setting the option +:instance_writer+ to false, as in
+The generation of the writer instance method can be prevented by setting the option +:instance_writer+ to +false+.
<ruby>
module ActiveRecord
@@ -1009,8 +1009,20 @@ end
A model may find that option useful as a way to prevent mass-assignment from setting the attribute.
+The generation of the reader instance method can be prevented by setting the option +:instance_reader+ to +false+.
+
+<ruby>
+class A
+ class_attribute :x, :instance_reader => false
+end
+
+A.x = 1 # NoMethodError
+</ruby>
+
For convenience +class_attribute+ also defines an instance predicate which is the double negation of what the instance reader returns. In the examples above it would be called +x?+.
+When +:instance_reader+ is +false+, the instance predicate returns a +NoMethodError+ just like the reader method.
+
NOTE: Defined in +active_support/core_ext/class/attribute.rb+
h5. +cattr_reader+, +cattr_writer+, and +cattr_accessor+

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