UniquenessValidator to be used outside the model (e.g. in form object)

uniqness_validator.rb

# Class UniquenessValidator adds 'model' option to the options so it can be used
# outside the model.
#
# See http://stackoverflow.com/a/14671979/807647
#
# Example:
#
#   class SignupForm
#     include ActiveModel::Model
#     attr_reader :email
#     validates :email,
#        uniqueness: { case_sensitive: false, model: User, attribute: 'name' }
#    end
#
class UniquenessValidator < ActiveRecord::Validations::UniquenessValidator

  def validate_each(record, attribute, value)
    # UniquenessValidator can't be used outside of ActiveRecord instances, here
    # we return the exact same error, unless the 'model' option is given.
    if !options[:model] && !record.class.ancestors.include?(ActiveRecord::Base)
      raise ArgumentError, "Unknown validator: 'UniquenessValidator'"

    # If we're inside an ActiveRecord class, and `model` isn't set, use the
    # default behaviour of the validator.
    elsif !options[:model]
      super

    # Custom validator options. The validator can be called in any class, as
    # long as it includes `ActiveModel::Validations`. You can tell the validator
    # which ActiveRecord based class to check against, using the `model`
    # option. Also, if you are using a different attribute name, you can set the
    # correct one for the ActiveRecord class using the `attribute` option.
    else
      record_orig, attribute_orig = record, attribute

      attribute = options[:attribute].to_sym if options[:attribute]
      record = options[:model].new(attribute => value)

      super

      if record.errors.any?
        options_orig = options.except(:case_sensitive, :scope).merge(value: value)
        record_orig.errors.add(attribute_orig, :taken, options_orig)
      end
    end
  end

end