form.html.erb

<%= form_for @serialized_products_form, url: serialized_products_path do |f| %>
  <%= f.fields_for :serialized_products, @serialized_products_form.serialized_products do |p| %>

      <%= p.label :product_id %><br>
      <%= p.text_field :product_id %>

      <%= p.label :location_id %><br>
      <%= p.text_field :location_id %>

      <%= p.label :serial %><br>
      <%= p.text_field :serial %>

  <% end %>
  
  <%= submit_tag %>
  
<% end %>

serialized_product_controller.rb

class SerializedProductsController < ApplicationController
  before_action :set_serialized_product, only: [:show, :edit, :update, :destroy]

  def new
    if session[:reference] == nil or session[:location_id] == nil or session[:line_item_ids] == nil
      print "\n\nSession is nil.\n\n"
      redirect_back(fallback_location: root_path)
    end
 
    @reference = session[:reference]
    location_id = session[:location_id]
    @line_items = InventoryAdjustmentItem.where(id: session[:line_item_ids])
    @serialized_products = []
    @line_items.each do |line_item|
      (line_item.quantity).times do 
        @serialized_products.push(SerializedProduct.new(product_id: line_item.product_id, location_id: location_id))
      end
    end
  end

  def create
    params["serialized_products"].each do |serialized_product|
      if serialized_product["serial"].present?
        SerializedProduct.create(serialized_product_params(serialized_product))
      end
    end
    redirect_to :root, notice: 'Serialized product was successfully created.'
    session[:reference] = nil
    session[:location_id] = nil
    session[:line_item_ids] = nil
  end

  private
    def set_serialized_product
      @serialized_product = SerializedProduct.find(params[:id])
    end
    
    def serialized_product_params(my_params)
      my_params.permit(:product_id, :location_id, :serial)
    end
end

serialized_products_form.rb

class SerializedProductsForm
  attr_reader :serialized_products
  def initialize(serialized_products:)
    @serialized_products = serialized_products
  end
  
  def serialized_products_attributes=
    ...
  end
end