validate uniqueness of (item type, size, category) triple in inventory

[juliagrigni]

I don't know why this doesn't work, but it allows duplicates and then errors when I try to edit the item to make it not a duplicate anymore: https://stackoverflow.com/questions/7428872/rails-validate-unique-combination-of-3-columns
in item.rb:
validates_uniqueness_of :itemType, scope: [:category, :size], message: "That category, name, size combination is already in use"