Rails 3 direct column reader

Whilst trying to get my head around arel and it’s relationship to ActiveRecord in rails 3, I’ve updated the simple ColumnReader class I introduced last year. It lets you read the (correctly cast) column values for an ActiveRecord class, without the overhead of instantiating each object.

Here’s the updated code:

module ColumnReader
  def column_reader(column_name, options = {})
    name = options.delete(:as) || column_name.to_s.pluralize
    column = columns_hash[column_name.to_s]

    self.module_eval %{
      def self.#{name}
        query = scoped.arel.project(arel_table[:#{column_name}])
        connection.select_all(query.to_sql).collect do |value|
          v = value.values.first
          #{column.type_cast_code('v')}
        end
      end
    }
  end

  ActiveRecord::Base.extend(self)
end

The code isn’t that different, though using scoped over construct_finder_sql feels a lot nicer. If you’ve got suggestions for improvement gist away.

Usage is similar to before, only using the new rails 3 syntax:

class Animal < ActiveRecord::Base
  column_reader 'id'
  column_reader 'name'

  named_scope :dangerous, :conditions => {:carnivorous => true}
end

Animal.names
#=> ['Lion', 'Tiger', 'Zebra', 'Gazelle']

Animal.limit(1).names
#=> ['Lion'] (Normal finder options supported)

Animal.dangerous.names
#=> ['Lion', 'Tiger'] (Scoping respected)

Animal.ids
#=> [1, 2, 3] (Values cast correctly)

I’m still not entirely convinced of the value of this helper, so if you find a good use tweet me. Enjoy!