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POActiveRecord Rails 3 scope vs class method
 

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    <p>I'm new to the new query interface of ActiveRecord so I'm still figuring things out. </p> <p>I was hoping someone could explain the difference between using a <code>scope</code> in an ActiveRecord model and just using a class method (ie <code>self.some_method</code>)</p> <p>From what I can gather, a scope is always expected to return a relation, whereas a class method doesn't necessarily have to. Is this true?</p> <p>For instance, I thought it would make sense to do something like:</p> <pre><code>class Person scope :grouped_counts, group(:name).count end </code></pre> <p>But this doesn't work. I get this error:</p> <pre><code>ArgumentError: Unknown key(s): communicating, failed, matched, unmatched from /Users/bradrobertson/.rvm/gems/ruby-1.9.2-p180@influitive/gems/activesupport-3.0.5/lib/active_support/core_ext/hash/keys.rb:43:in `assert_valid_keys' from /Users/bradrobertson/.rvm/gems/ruby-1.9.2-p180@influitive/gems/activerecord-3.0.5/lib/active_record/relation/spawn_methods.rb:110:in `apply_finder_options' from /Users/bradrobertson/.rvm/gems/ruby-1.9.2-p180@influitive/gems/activerecord-3.0.5/lib/active_record/named_scope.rb:110:in `block in scope' from (irb):48 from /Users/bradrobertson/.rvm/gems/ruby-1.9.2-p180@influitive/gems/railties-3.0.5/lib/rails/commands/console.rb:44:in `start' from /Users/bradrobertson/.rvm/gems/ruby-1.9.2-p180@influitive/gems/railties-3.0.5/lib/rails/commands/console.rb:8:in `start' from /Users/bradrobertson/.rvm/gems/ruby-1.9.2-p180@influitive/gems/railties-3.0.5/lib/rails/commands.rb:23:in `&lt;top (required)&gt;' from script/rails:6:in `require' from script/rails:6:in `&lt;main&gt;' r </code></pre> <p>It does however work as a class method</p> <pre><code>def self.grouped_counts group(:name).count end </code></pre> <p>I'm interested to know peoples' thoughts on when to use scopes and when to use class methods. Am I correct in assuming that a scope must always return a relation, but a class method can return whatever it wants?</p>
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