Question

I understand that validating uniqueness of a standard, single field like “username” is easy. However, for something that has an unlimited number of inputs like, for example, “Favorite Movies” where a user can add as many favorite movies, is something I can’t figure out.

They can choose to add or remove fields via the builder, but how do I ensure that no two or more entries are duplicates?

Answer 1

It turns out that when using nested attributes, you can only validate what’s already in the database and not new duplicate occurrences. So, a validation extension (below) with memory validation is really the only option, unfortunately.

#user.rb

class User
  has_many :favorite_movies

  validate :validate_unique_movies

  def validate_unique_movies
    validate_uniqueness_of_in_memory(
      favorite_movies, [:name, :user_id], 'Duplicate movie.')
  end
end

#lib/extensions.rb

module ActiveRecord
  class Base
    def validate_uniqueness_of_in_memory(collection, attrs, message)
      hashes = collection.inject({}) do |hash, record|
        key = attrs.map {|a| record.send(a).to_s }.join
        if key.blank? || record.marked_for_destruction?
          key = record.object_id
        end
        hash[key] = record unless hash[key]
        hash
      end
      if collection.length > hashes.length
        self.errors.add_to_base(message)
      end
    end
  end
end