cute_validation_example.rb

# in the model
validates_with CoordinatesValidator, :fields => [:location]


# Error messaging needs improvement. Since this validator will only whine if someone is forging requests
# with invalid data, i'm leaving it as it is.

class CoordinatesValidator < ActiveModel::Validator
  def validate(record)
    if options[:fields].any? { |f| invalid_latlng?(record.send(f)) }
      record.errors[:base] << "Invalid latlng value"
    end
  end

  private
  def invalid_latlng?(value)
    return true if value.size != 2
    !(value[0].is_a?(Float) && value[1].is_a?(Float))
  end
end

Thanks a lot!. This works very well and it is easily extendable.