How to get most recent single item from many with ActiveRecord associations?

Question

I'm having some issues with my code. This is probably due to some design fault. I've tried several things. Here are two.

Issue: I have a 1 (lending) - N (contracts) relation. I'd like to call lending.current_contract which would return the last relevant contract. Optimization is also an issue. I'd like to call :lending, :include => :contracts without having to use separate queries for contracts which are already in the contracts array containing all contracts.

Bad solution 1:

has_one :current_contract, :class_name => "Contract"

This doesn't work because every time I create, update or destroy it has to update the parent lending as well. Doing this I get a callback mess. For example when creating a lending, it also creates the first contract. For some reason going back and forth using callbacks for both the lending as its contract doesn't work.

Bad solution 2:

def current_contract
  return if contracts.relevant.empty?
  @current_contract = contracts.relevant.last
end

Sends a copy and not the reference. So lending.current_contract.status = value doesn't work.

Is there some design pattern which I should look at? And are there some examples? I've looked at some github projects but none of them had similar problems solved, therefore I think it is a design problem.

Answer 1

Associations can generally take a :conditions hash, which can be handy. (And I'd forgotten this until I needed it about half an hour ago).

Would it help? Something like:

has_one :current_contract, :class_name => "Contract", :conditions => ...

Looking at the book a bit more (page 364 to be exact):

has_one :current_contract, :class_name => "Contract", :order => 'created_at DESC'

...which will reference the most recently-created contract. You may, of course, have a more suitable column.

I wish I'd seen that earlier - I need to go and change some code now...

Answer 2

By 'relevant', do you mean most recent?

class Lending < ActiveRecord::Base
  has_many :contract
  attr_reader :current_contract

  def initialize
   @current_contract = Contract.New
  end
  ...
end

class Contract < ActiveRecord::Base
  has_one :lending
  ...

  def before_delete
    # update lending to the most relevant contract
    # if this is the current_contract for parent lending
  end
end