Ruby on Rails has_many through association objects before save

on a Ruby on Rails project I'm trying to access association objects on an ActiveRecord prior to saving everything to the database.

class Purchase < ActiveRecord::Base

  has_many :purchase_items, dependent: :destroy
  has_many :items, through: :purchase_items

  validate :item_validation

  def item_ids=(ids)
    ids.each do |item_id|
      purchase_items.build(item_id: item_id)
    end
  end

  private

  def item_validation
    items.each do |item|
      ## Lookup something with the item
      if item.check_something
        errors.add :base, "Error message"
      end
    end
  end

end

If I build out my object like so: purchase = Purchase.new(item_ids: [1, 2, 3]) and try to save it the item_validation method doesn't have the items collection populated yet, so even though items have been set set it doesn't get a chance to call the check_something method on any of them.

Is it possible to access the items collection before my purchase model and association models are persisted so that I can run validations against them?

If I change my item_validation method to be:

def item_validation
  purchase_items.each do |purchase_item|
    item = purchase_item.item
    ## Lookup something with the item
    if item.something
       errors.add :base, "Error message"
    end
  end
end

it seems to work the way I want it to, however I find it hard to believe that there is no way to directly access the items collection with rails prior to my purchase and associated records being saved to the database.