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    <p>There's a lot of info missing to properly answer this. The test implies that <code>find_by_artist</code> is always defined even when <code>@library</code> is empty, but that there are dynamic methods available on other attributes (eg: bitrate) that are valid only when library contains a record with such a method.</p> <p>One should not redefine <code>respond_to?</code> in any case. There is an explicit hook method for answering respond_to? for dynamic methods: <a href="http://ruby-doc.org/core-1.9.3/Object.html#method-i-respond_to_missing-3F" rel="nofollow"><code>Object#respond_to_missing?</code></a>.</p> <p>So a simple way to make your test pass is to be sure the @library object has a concrete method <code>#find_by_artist</code> and a respond to hook that checks whether any of it's elements a have the requested attribute. If I assume @library is a collection object <code>Library</code> which keeps an enumeration of songs in <code>@songs</code></p> <pre><code>class Library def find_by_artist artist @songs.select { |song| song['artist'] == artist } end def method_missing meth, arg m = /^find_by_(.+)$/.match meth.to_s return super unless attr = m &amp;&amp; m[1] @songs.select { |song| song[attr] == arg } end def respond_to_missing? meth, include_private m = /^find_by_(.+)$/.match meth.to_s return super unless attr = m &amp;&amp; m[1] @songs.any? { |song| song.has_key? attr } end end </code></pre> <p>This has a performance problem in that respond_to? now incurs a search of all the songs. One could optimize by keeping a set of the union of all attributes contained in @songs and updating it in methods which add/update/delete elements in the collection.</p>
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