class Shadow
     has_and_belongs_to_many :interests, :class_name => 'Product'
end

class Product
       has_and_belongs_to_many :viewers, :class_name => 'Shadow'
end

# This works
Shadow.where(:created_at.lte => Time.now).count
=> 502

# This doesn't
Product.first.viewers.where(:created_at.lte => Time.now).count
 => 0