populated_collection_validator.rb

# Validates that a given collection is properly populated.
#
# Usage:
#
#     class Person
#       has_many :assets
#       validates :assets, :populated_collection => true
#    end
#
class PopulatedCollectionValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)
    keep_results = value.select { |o| !o.marked_for_destruction? }
    record.errors.add(attribute, options[:message] || "can't be empty.") if keep_results.empty?
  end
end