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    <p>I just stumbled onto this question while running into the same problem.</p> <p>For reference, the <a href="http://apidock.com/rails/Module/attr_accessor_with_default" rel="nofollow">docs</a> specify the block form is dynamically evaluated in the instance scope. To continue the example, its usefulness is really quite limited, but at least works the way you might expect:</p> <pre><code>class Container attr_accessor_with_default(:block_collection) { name.underscore } end &gt; c = Container.new(:name =&gt; "TestName") &gt; c.block_collection # =&gt; "test_name" &gt; c.block_collection = "something else" # =&gt; "something else" &gt; c.name =&gt; "TestName" </code></pre> <p>Here's the really quirky part though...</p> <pre><code>class Container attr_accessor_with_default :naive_collection, [0, 0] end # This works as expected &gt; c = Container.new &gt; c.naive_collection = ["foo", "bar"] # =&gt; ["foo", "bar"] &gt; Container.new.naive_collection # =&gt; [0, 0] &gt; c.naive_collection[0] = 0 # =&gt; [0, "bar"] &gt; Container.new.naive_collection # =&gt; [0, 0] # But this doesn't &gt; c2 = Container.new &gt; c2.naive_collection # =&gt; [0, 0] &gt; c2.naive_collection[0] = "problem!" # =&gt; ["problem!", 0] &gt; Container.new.naive_collection # =&gt; ["problem!", 0] </code></pre> <p>Digging into the source a bit, I see that attr_accessor_with_default defines an instance method that returns the default value or executes the block. Which is fine.</p> <p>It then goes on to execute this within a module_eval:</p> <pre><code>def #{sym}=(value) class &lt;&lt; self; attr_reader :#{sym} end @#{sym} = value end </code></pre> <p>Which is just ridiculously convoluted. I may end up ticketing this as a bug if I can't figure out the motivation for this behavior.</p> <p><strong>Update:</strong></p> <p>I managed to figure out what was going wrong here.</p> <pre><code>def attr_accessor_with_default(sym, default = Proc.new) define_method(sym, block_given? ? default : Proc.new { default }) module_eval(&lt;&lt;-EVAL, __FILE__, __LINE__ + 1) def #{sym}=(value) class &lt;&lt; self; attr_accessor :#{sym} end @#{sym} = value end EVAL end </code></pre> <p>Initially, the default value exists as a proc. Once you invoke the setter, the getter and setter methods are overwritten by attr_accessor methods and the instance variable is initialized. The problem is that the default proc in the getter returns the class-level default value. So when you do something like:</p> <pre><code>&gt; c2.naive_collection[0] = "problem!" # =&gt; ["problem!", 0] </code></pre> <p>you're actually changing the default value for the class.</p> <p>I think this method should probably be implemented as:</p> <pre><code>class Module def attr_accessor_with_default(sym, default = Proc.new) module_eval(&lt;&lt;-EVAL, __FILE__, __LINE__ + 1) def #{sym} class &lt;&lt; self; attr_reader :#{sym} end @#{sym} = #{ default.dup } end def #{sym}=(value) class &lt;&lt; self; attr_accessor :#{sym} end @#{sym} = value end EVAL end end </code></pre> <p>I'll ticket it and offer up a patch.</p> <p><strong>Update again:</strong> <a href="https://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/6496" rel="nofollow">https://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/6496</a></p>
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