Rails: Validating uniqueness without saving

uniqueness_validation.rb

# Had to do this for an importer tool that was being converted to a three-step validation-feedback-persist process
# Rails does not support uniqueness validation for a custom collection of unsaved ActiveRecord models
# Maybe someone finds this useful

def validate_uniqueness
  ActiveRecord::Base.transaction do
    @unsaved_entities.each do |entity|
      entity.object.save # Executes validations/callbacks
      entity.successful = entity.object.valid?
    end
    # Roll back all changes
    raise ActiveRecord::Rollback
  end
end

Had to do this for an importer tool that was being converted to a three-step validation-feedback-persist process.
Rails does not support uniqueness validation for a custom collection of unsaved ActiveRecord models.