Using jbuilder in rails to construct an array of objects with different classes

A model(FirstModel) in my rails app has a has_many relationship with another model(Item).

class FirstModel < ActiveRecord::Base

  has_many :items, :dependent => :destroy

  def item_array
    a = []
    self.items.each do |item|
      a.push(item.item_thing)
    end
    a
  end
end

Item itself belongs :item_thing through a polymorphic association. A method on FirstModel returns an array of "items", not the join model object(which is of class Item), but the item_thing of each Item object, and therefore it is an array of objects of different classes (checked in rails console, and the method returns the array just fine)

class Item < ActiveRecord::Base
  belongs_to :item_thing,  :polymorphic => true


end

In my show.json.jbuilder for FirstModel, I want to take that array of "items" and use different templates based on which class the item belongs to. The following file path is "views/api/items/show.jbuilder"

json.extract! @item, :name

json.items @item.item_array do |item|
  if item.class == 'Book'
    json.partial! 'api/books/show', item
  end
  if item.class == 'Car'
    json.partial! 'api/cars/show', item
  end
end

The car and book template paths are "views/api/cars/show.json.jbuilder" and "views/api/books/show.json.jbuilder.

When I get the json from the above template the name of the FirstModel object is visible, items array is empty, whereas I am trying to get an array of hashes for each item, rendered through its respective template.

Thank you for helping!