Problem with has_many :through, :uniq => true with polymorph

Didn't have quite enough space to describe it there...basically i'm having a problem with the :uniq option in my tags.

I'm using acts_as_taggable_on_steroid which adds these associations to my Resource class:

Resource   has_many :taggings, :as => :taggable, :dependent => :destroy, :include => :tag   has_many :tags, :through => :taggings, :uniq => true

This uses the join model Tagging:

Tagging   belongs_to :tag   belongs_to :taggable, :polymorphic => true

And of course the Tag model:

Tag   has_many :taggings, :dependent => :destroy   has_many_polymorphs :taggables,     :from => [:resources, :lessons],     :through => :taggings,     :dependent => :destroy

The uniq option doesn't seem to stop me from adding the same tag multiple times to a resource:

resource = Resource.first

=> #<TeachingObject id: 59, name: "Clarinet bite point close-up" ...>

tag = Tag.first

=> #<Tag id: 1, name: "recorder">

resource.tags

=> [#<Tag id: 1, name: "recorder">, #<Tag id: 610, name: "chunky chicken">, #<Tag id: 1, name: "recorder">]

resource.tags << tag

=> [#<Tag id: 1, name: "recorder">, #<Tag id: 610, name: "chunky chicken">, #<Tag id: 1, name: "recorder">, #<Tag id: 1, name: "recorder">]

resource.tags << tag

=> [#<Tag id: 1, name: "recorder">, #<Tag id: 610, name: "chunky chicken">, #<Tag id: 1, name: "recorder">, #<Tag id: 1, name: "recorder">, #<Tag id: 1, name: "recorder">]

Now, i have other classes which use has_many :through with :uniq => true and they seem fine: for example. resources also join to assets, through a table called 'items':

Resource   has_many :items, :dependent => :destroy, :order => :position   has_many :assets, :through => :items, :order => :position, :uniq => true

Item   belongs_to :asset   belongs_to :resource

Asset   has_many :items, :dependent => :destroy, :order => :position   has_many :resources, :through => :items, :order => :position

Like i say, this behaves as i'd expect:

resource = Resource.first

=> #<TeachingObject id: 59, name: "Clarinet bite point close-up" ... >

asset = Asset.first

=> #<Asset id: 1, name: "07 Clari Clari (notation, perf AI)" ... >

resource.assets

=>

resource.assets << asset

=> [#<Asset id: 1, name: "07 Clari Clari (notation, perf AI)" ... >]

resource.assets << asset

=> [#<Asset id: 1, name: "07 Clari Clari (notation, perf AI)" ... >]

See? It doesn't add the duplicate.

I'm wondering if it's to do with the polymorphic nature of the tag - resource relationship - maybe the polymorphism is stopping it from noticing that it's not uniq? or something?

Grateful for any help or ideas...i can't even think of a way to work around this since the problem is buried inside active record's << method.