gistfile1.rb

# real world example in zena: http://github.com/zena/zena/blob/master/lib/zena/acts/secure.rb

# in a module that you include where you need to find
def filter_fields(klass)
  scope = {}
  find_scope = scope[:find] = {}
  find_scope[:select] = klass.fields_for(Thread.current.visitor).map {|f| "#{klass.table_name}.#{f}"}.join(',')
  klass.send(:with_scope, scope) { yield }
end

# in the class
class Product < ActiveRecord::Base
  def self.fields_for(visitor)
    if visitor.is_admin?
      %w{*}
    else
      %w{name price category}
    end
  end
end

# usage
products = filter_fields(Product) { Product.find(:all, :conditions => ['price > ?', params[:price]]) }

# if you do not like this syntax, you could also create a custom finder:

class Product < ActiveRecord::Base
  def self.filter_find(*args)
    filter_fields(self) { self.find(*args) }
  end
end