How do I find the .max of an attribute value among a group of different Models?

everyone: I am also open to just straight-up refactoring what I'm finding to be pretty repetitive, but to give a baseline of how it's working....

I have for every contact a Campaign, which has_many of three types of Models: Email, Call, and Letter.

When an Email (Call or Letter) has been executed for a specific contact, I have a Contact_Email(_or_Call_or_Letter) which belongs to both the Contact and the Model (Email_or_Call_or_Letter).

Each Contact_Email for example pairing has a :date_sent attribute. So does each Contact_Call and Contact_Letter.

How do I find the latest of all of them?

Here is the code I wrote that can find the latest Email and my finding retyping similar code for Call and Letter, but then stuck on how to do a .max on all of them:

  def last_email(contact)
    #get campaign the contact belongs to
    @campaign = Campaign.find_by_id(contact.campaign_id)

    @last_email = ContactEmail.find(:last, 
                        :conditions => "contact_id = #{contact.id}",
                        :order => "date_sent DESC")

    @last_call = ContactCall.find(:last, 
                        :conditions => "contact_id = #{contact.id}",
                        :order => "date_sent DESC")

    @last_letter = ContactLetter.find(:last, 
                        :conditions => "contact_id = #{contact.id}",
                        :order => "date_sent DES开发者_StackOverflow社区C")

    # how do I get the latest of all of these to display?

    @email_template = Email.find_by_id(@last_email.email_id)

    if @last_email.nil?
      return "no email sent"
    else
      return @last_email.date_sent.to_s(:long) + link_to('email was sent', @email_template)
    end
  end

Question 1: With what I have, how can I find effectively @last_event given I can find the last Email, last Call, and last Letter for every contact?

Question 2: How can I remove the repetitive code that I have to write for each Model?

---

Do you have has_many associations setup in Contact referring to the other models? Something like:

class Contact < ActiveRecord::Base
  has_many :contact_emails
  has_many :contact_calls
  has_many :contact_letters
end

If so, you can then create a last_event method on the Contact model:

def latest_event
  [contact_emails, contact_calls, contact_letters].map do |assoc|
    assoc.first(:order => 'date_sent DESC')
  end.compact.sort_by { |e| e.date_sent }.last
end

---

Handling nil

When using the latest_event method you will get nil if there are no associated records. There are a couple of ways you can workaround this. The first is to check for nil first with something like:

contact.latest_event && contact.latest_event.date_sent

On late versions of Rails/Ruby you can also use Object#try which will call the method if it exists:

contact.latest_event.try(:date_sent)

I prefer not to use this as it doesn't check for nil but only if the object can respond to a method. This has cause some interesting errors if you expect nil if the object is nil but are calling a method which nil itself responds to.

Finally, my preferred method for the simple case is to use the andand gem which provides Object#andand. This greatly shortens the safe case above and saves calling of latest_event multiple times:

contact.latest_event.andand.date_sent

date_sent, nil and You.

For your example usage of calling to_s(:long), you could either use && or andand:

contact.latest_event.andand.date_sent.andand.to_s(:long)

or

contact.latest_event && contact.latest_event.date_sent.to_s(:long)

The first is safer if date_sent itself may be nil. Without using andand this could be written as:

contact.latest_event &&
  contact.latest_event.date_sent &&
  contact.latest_event.date_sent.to_s(:long)

which is rather complex and unwieldily in my opinion. I would recommend looking into andand

---

For question 1:

Just do

@last_event = [@last_letter, @last_email, @last_call].sort_by{|m| m.date_sent}.first

For question 2:

Well this is more interesting. This kind of depends on how exactly do your models look, but you might want to consider Single Table Inheritance for this type of scenario.