Rails preventing duplicates in polymorphic has_many :through associations

Is there an easy or at least elegant way to prevent duplicate entries in polymorphic has_many through associations?

I've got two models, stories and links that can be tagged. I'm making a conscious decision to not use a plugin here. I want to actually understand everything that's going on and not be dependent on someone else's code that I don't fully grasp.

To see what my question is getting at, if I run the following in the console (assuming the story and tag objects exist in the database already)

s = Story.find_by_id(1)

t = Tag.find_by_id(1)

s.tags << t

s.tags << t

My taggings join table will have two entries added to it, each with the same exact data (tag_id = 1, taggable_id = 1, taggable_type = "Story"). That just doesn't seem very proper to me. So in an attempt to prevent this from happening I added the following to my Tagging model:

before_validation :validate_uniqueness

def validate_uniqueness
    taggings = Tagging.find(:all, :conditions => { :tag_id => self.tag_id, :taggable_id => self.taggable_id, :taggable_type => self.taggable_type })

    if !taggings.empty?
        return false
    end

    return true
end

And it works almost as intended, but if I attempt to add a duplicate tag to a开发者_运维百科 story or link I get an ActiveRecord::RecordInvalid: Validation failed exception. It seems that when you add an association to a list it calls the save! (rather than save sans !) method which raises exceptions if something goes wrong rather than just returning false. That isn't quite what I want to happen. I suppose I can surround any attempts to add new tags with a try/catch but that goes against the idea that you shouldn't expect your exceptions and this is something I fully expect to happen.

Is there a better way of doing this that won't raise exceptions when all I want to do is just silently not save the object to the database because a duplicate exists?

You could do it a couple of ways.

Define a custom add_tags method that loads all the existing tags then checks for and only adds the new ones.

Example:

def add_tags *new_tags
  new_tags = new_tags.first if tags[0].kind_of? Enumerable #deal with Array as first argument
  new_tags.delete_if do |new_tag|
    self.tags.any? {|tag| tag.name == new_tag.name}
  end
  self.tags += new_tags
end

You could also use a before_save filter to ensure that the list of tags doesn't have any duplicates. This would incur a little more overhead because it would happen on EVERY save.

You can set the uniq option when defining has_many relation. Rails API docs says:

:uniq

If true, duplicates will be omitted from the collection. Useful in conjunction with :through.

(taken from: http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#M001833 under "Supported options" subheading)

I believe this works...

class Tagging < ActiveRecord::Base
   validate :validate_uniqueness 

   def validate_uniqueness
      taggings = Tagging.find(:all, :conditions => {
         :tag_id => self.tag_id,
         :taggable_id => self.taggable_id,
         :taggable_type => self.taggable_type }) 

      errors.add_to_base("Your error message") unless taggings.empty? 
   end 
end

Let me know if you get any errors or something with that :]