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    <p>You are asking the difference between a class method and an instance method.</p> <p>There are a few ways of defining a class method:</p> <pre><code>class Klass def Klass.method_name .. end end </code></pre> <p>which is the same as doing:</p> <pre><code>class Klass def self.method_name .. end end </code></pre> <p>or the preferred ruby idiom:</p> <pre><code>class Klass class &lt;&lt; self def method_name .. end end end </code></pre> <p>If Klass is already declared you can also do..</p> <pre><code>def Klass.method_name .. end </code></pre> <p>or: </p> <pre><code>class &lt;&lt; Klass def method_name .. end end </code></pre> <p>or you could even use Module#extend:</p> <pre><code>Klass.extend(Module.new { def method_name; puts 'ducky'; end }) </code></pre> <p>Much like you would add a singleton method to an object. In fact class methods are singleton methods which operate on the class level.</p> <p>In rails ActiveRecord for example you have a class method 'find' which you can use on any Model: </p> <pre><code>Person.find(1) </code></pre> <p>and instance methods like 'save' which operates on the individual object</p> <pre><code>person = Person.find(1) ... person.save </code></pre> <p>In the current project I am working on I have a model Feed which contains data feeds. Periodically I need to run a method which updates all the feeds so i have an fetch_all method which accomplishes this.</p> <pre><code>class Feed &lt; ActiveRecord::Base // class method Feed.fetch_all def self.fetch_all Feed.all.each do |feed| feed.fetch_value end end // instance method def fetch_value // grabs updated value and saves end end </code></pre>
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