Rails - Dec 2005 - validates_uniqueness_of on has_and_belongs_to_many

I am trying to create a model that allows only unique assets to be
added to a category. However, they could be added again to a different
category. Finally, an asset can be shared across many categories. So
my question is how do you check the uniqueness across a join table?

This does not compile....

has_and_belongs_to_many :categories, :foreign_key =>
''asset_id''
validates_uniqueness_of :data, :scope =>
''asset_categories.category_id''


Thanks,
Mark

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I am Mark Daggett and I approve this message.