I&#39;m new to the new query interface of ActiveRecord so I&#39;m still figuring things out.  

I was hoping someone could explain the difference between using a `scope` in an ActiveRecord model and just using a class method (ie `self.some_method`)

From what I can gather, a scope is always expected to return a relation, whereas a class method doesn&#39;t necessarily have to.  Is this true?

For instance, I thought it would make sense to do something like:

    class Person
      scope :grouped_counts, group(:name).count
    end

But this doesn&#39;t work.  I get this error:

    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&#39;
    	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&#39;
    	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&#39;
    	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&#39;
    	from /Users/bradrobertson/.rvm/gems/ruby-1.9.2-p180@influitive/gems/railties-3.0.5/lib/rails/commands/console.rb:8:in `start&#39;
    	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;&#39;
    	from script/rails:6:in `require&#39;
    	from script/rails:6:in `&lt;main&gt;&#39;
    r

It does however work as a class method

    def self.grouped_counts
      group(:name).count
    end

I&#39;m interested to know peoples&#39; 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?

All Scopes Solutions on Gang of Coders

Total of 1 Scopes Solutions

ActiveRecord Rails 3 scope vs class method