Nested form/mass assignment, validations trouble, has_many_through

Question

Ok, so I'm having trouble understanding where I'm going wrong here. I have my associations set up correctly and my new order form works correctly (note: if both fields are filled out). If I only fill out one field, for a one item order, the mass assignment fails. I want to be able to eventually have 3 fields but if only 1 or 2 of them are filled out, everything goes through fine, with the empty fields being ignored. I've tried to understand this for hours and now I've tried everyway I can think of debugging/undertanding this thing.

The view/form: new.html.erb

<%= form_for @order do |f| %>
  <%= f.hidden_field :date, :value => Time.now %>
  <%= f.fields_for :order_items do |f| %>
    <%= f.collection_select :item_id, Item.all, :id, :item_type, :include_blank => true %>
    <%= f.label "Quantity: "%>
    <%= f.text_field :quantity %><br>
  <% end %>
  <%= f.submit %>
<% end %>

Controller actions:

def new
  @order = Order.new
  @order.order_items.build
  @order.order_items.build  #for 2 fields
end

def create
  @user = current_user
  @account = Account.find_or_create_by(user_id: @user.id)

  @account.orders.create(params_check)
end    

Models:

class Order < ApplicationRecord
  has_many :order_items
  has_many :items, through: :order_items
  belongs_to :account

  def order_items_attributes=(order_items_attributes)
    order_items_attributes.each do |i, order_item_attributes|
      #binding.pry
      self.order_items.build(order_item_attributes)
    end
  end
end

class OrderItem < ApplicationRecord
  belongs_to :order
  belongs_to :item

  **NOTE: ALL THESE VALIDATIONS SEEM TO BE USELESS**
  #validates :item_id, :presence => {:message => "You must choose"}
  #validates :quantity, :presence => {:message => "You must choose"}
  #validates :order_id, :presence => {:message => "You must choose"}
  #validates_presence_of :item_id, :message => "You must choose an item"
  #validates_presence_of :quantity, :message => "You must enter the quantity"
end

class User < ApplicationRecord
  # Include default devise modules. Others available are:
  # :confirmable, :lockable, :timeoutable and :omniauthable
  devise :database_authenticatable, :registerable,
     :recoverable, :rememberable, :trackable, :validatable

  has_one :account
  has_many :orders, through: :account
end

class Account < ApplicationRecord
  belongs_to :user
  has_many :orders, dependent: :destroy
end

class Item < ApplicationRecord
  #belongs_to :order
  has_many :order_items
end

I hope that is all clear. I'm still a beginner so any help would be appreciated! Thanks