Find condition needs refactoring/optimising on polymorphic association

I'm building a recommendation method for users in my project. Users generate interest records whenever they view, create, comment or interact with objects (weighted depending on the action).

I've written a find method that looks at a user's interests, and then finds users who are also interested in those items. However, it is horrendously inefficient, making as many db calls as the user has interests (up to 50).

Here's a chopped down version of what's going on:

#User.rb
...
has_many :interests, :as => :interestable, :dependent => :destroy

def recommendations
  recommendations = []

  Interest.for(self).limit(50).each do |item|
    recommendations << Interest.other_fans_of(item)
  end

  user_ids = recommendations.flatten.map(&:user_id).uniq
end
...

#interest.rb

...

belongs_to :user
belongs_to :interestable, :polymorphic => true

named_scope :for, lambda { |user| { :conditions => { :user_id => user.id } } }
named_scope :limit, lambda { |num| { :limit => num } }
named_scope :other_fans_of, lambda { |interest| { :conditions => { :interestable_t开发者_如何学Cype => interest.interestable_type, :interestable_id => interest.interestable_id } } }
default_scope :order => "weight DESC"

...

Are there any sql geniuses out there who can turn that into one nice clean db call?

---

Something like this should do the job. There might be prettier ways…

class User < ActiveRecord::Base
  #...
  def recommendations
    # get a list of the relevant interests
    the_interests = Interest.for(self).limit(50).map{|x| [x.interestable_type, x.interestable_id]}
    # make some sql
    conditions = the_interests.map{|x| "(`interestable_type`=? AND `interestable_id`=?)"}.join(" OR ")
    # use ruby magic to make a valid finder and get the other user_ids
    user_ids = Interest.all(:select => '`user_id`', :conditions => [conditions, *(the_interests.flatten)]).map(&:user_id).uniq
  end
  #...
end