In an application I’m working on, people can leave comments on a record. When they comment, they can also choose to email that comment to their co-workers. The client wanted the application to validate these email addresses and show an error if the commenter entered a bad email address.

The email addresses aren’t persisted. No reason to, as they are only used when sending out the email. So, there’s no ActiveRecord model for CommentRecipent or anything like that. But the nicety of ActiveRecord-style validation was exactly what I was looking for. All I wanted the comment controller to do was something like this:

def create
  ...
  if comment.save?
    # Send emails and notify the commenter of success
  else
    # notify commenter of problems with comment
  end
end

Now, probably the most thorough way to approach this would be to create a plain-old-Ruby-object or two and get them to pass the ActiveRecord Lint tests and then have them use ActiveRecord validation. It works, and I’ve done that in the past. But this time I aimed for something a little lighter weight. What I wanted was for the Comment model to have these lines:

validates_associated :recipients

def recipients
  @recipients ||= CommentRecipients.new
end

And then a comment could manage its recipients with an API like:

@comment.recipients.add "someemail@test.com"
@comment.recipients.valid?
@comment.recipients.each do {|recipient| send_email_to(recipient)}

That’s pretty much all I needed.

Looking at the validates_associated documentation, I see that it eventually calls this code:

if Array.wrap(value).reject {|r| r.marked_for_destruction? || r.valid?}.any?
  record.errors.add(attribute, :invalid, options.merge(:value => value))
end

So for my recipients object it needs to respond to valid? and marked_for_destruction?. Valid? makes sense, but marked_for_destruction?, what is that? Here’s the Rails documentaion description:

Returns whether or not this record will be destroyed as part of the parents save transaction.

Since recipients are never perisisted, they are never marked for destruction. So I can just have this return false. So, now I know most of the methods my recipients object will have to handle:

• valid?
• marked_for_destruction?
• each
• add

And here’s a first pass:

class CommentRecipients
  include Enumerable

  def add(address)
    recipients << address
  end

  def recipients
    @recipients ||= []
  end

  def each(&block)
    @recipients.each do |r|
      block.call(r)
    end
  end

  #This method is here just so Comment can use validates_associated
  def marked_for_destruction?
    false
  end

  def valid?
  end

  private
end

Including Enumerable lets me make this act like a collection. The collection of recipients lives in an Array (a Set might be better, actually), which grows by using add. I’ve defined #each do just iterate through the collection of recipients and I’ve hard-coded marked_for_destruction? to always return false. The only thing missing here is valid?

You could go about this a couple of ways. Define valid to iterate through @recipients and return false unless they all have valid email addresses. Or you could extract a CommentRecipent out to its own class. I did the latter. All the CommentRecipient has to respond to is new, valid and provide a way to set its address:

class CommentRecipient
  attr_accessor :address

  def initialize(address="")
    self.address = address
  end

  def address=(x)
    @address = x.strip
  end

  def valid?
    /^.+@.+$/.match(address)
  end
end

Not much to explain there. The only weird bit might be my valid? regex, as it’s much less complex than most email validation patterns. I went that way after reading this blog post about the hassle of regex-based email validation. This super-dumb pattern is good enough for now.

Introducing the new CommentRecipient introduces some changes to my Recipients collection:

class CommentRecipients
  include Enumerable

  def add(address)
    recipients << CommentRecipient.new(address)
  end

  ...

  def each(&block)
    @recipients.each do |r|
      block.call(r.address)
    end
  end

  ...

  def valid?
    invalid_recipients.empty?
  end

  private

  def invalid_recipients
    recipients.select {|r| !r.valid?}
  end
end

Normally that each definition would return each recipient, but I have it returning each address for the purpose of integrating with some other code that should probably be rewritten.

And this works. A comment now validates its recipients and won’t save if one of them is invalid. And the application returns this awesome error message:

Recipient is invalid

Well, that’s super useful! Let’s turn that into something a human can understand. I could define the error collection and set up the messages in there. Or I could use the Rails internationalization library, which is way easier for this one simple case. If I had a situation where a Recipient could be invalid for a variety of reasons I’d take the more robust errors approach. But in this situation I could just add the following to config/locals/en.yml

activerecord:
  attributes:
    comment:
      recipients: "Other recipients"
  errors:
    models:
      comment:
        attributes:
          recipients:
            invalid: "can only contain valid email addresses. Separate multiple addresses with a comma."

And now we have a much better validation message.

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