one_item_per_package.rb

module Spree
  module Stock
    module Splitter
      class OneItemPerPackage < Base
        cattr_accessor :shipping_category_names do
          []
        end

        def split(packages)
          packages_to_be_processed = packages.select do |package|
            package.contents.any? do |item|
              self.class.shipping_category_names.include?(item.line_item.variant.shipping_category.name)
            end
          end

          packages_to_be_processed.each do |package|
            packages.delete(package)
            package.contents.each do |item|
              item.quantity.times do
                package = build_package
                package.add(item.line_item, 1, item.state, item.variant)
                packages << package
              end
            end
          end

          packages
        end
      end
    end
  end
end

one_item_per_package_spec.rb

require 'spec_helper'

module Spree
  module Stock
    module Splitter
      describe OneItemPerPackage do
        let(:line_item1) do
          line_item = build(:line_item, variant: build(:variant))
          line_item.variant.product.shipping_category = create(:shipping_category, name: 'Mattress')
          line_item
        end

        before do
          Spree::Stock::Splitter::OneItemPerPackage.shipping_category_names = ["Mattress"]
        end

        let(:packer) { build(:stock_packer) }

        subject { Spree::Stock::Splitter::OneItemPerPackage.new(packer) }

        it "allows only one item in each package from the 'Mattress' shipping category" do
          package1 = Package.new(packer.stock_location, packer.order)
          package1.add line_item1, 2

          packages = subject.split([package1])
          expect(packages.count).to eq(2)
        end

      end
    end
  end
end