Rails idiom to avoid duplicates in has_many :through

As long as the appended role is an ActiveRecord object, what you are doing:

user.roles << role

Should de-duplicate automatically for :has_many associations.

For has_many :through, try:

class User
  has_many :roles, :through => :user_roles do
    def <<(new_item)
      super( Array(new_item) - proxy_association.owner.roles )
    end
  end
end

if super doesn’t work, you may need to set up an alias_method_chain.