```ruby
FactoryBot.define do
  factory :shipment_with, class: Spree::Shipment do
    tracking 'U10000'
    number '100'
    cost 100.00
    state 'pending'
    order
    address
    stock_location

    trait :shipping_method do
      transient do
        shipping_method { create(:shipping_method) }
      end

      shipping_rates {
        [Spree::ShippingRate.create(shipping_method: shipping_method, selected: true)]
      }

      after(:create) do |shipment, _evaluator|
        shipment.order.line_items.each do |line_item|
          line_item.quantity.times {
            shipment.inventory_units.create(variant_id: line_item.variant_id)
          }
        end
      end
    end
  end
end

FactoryBot.modify do
  factory :shipment, class: Spree::Shipment do
    # keeps test shipments unique per order
    initialize_with { Spree::Shipment.find_or_create_by(order_id: order.id) }
  end
end
```

Bring shipment factory from spree_core and merge with modification
user
```ruby
FactoryBot.define do
  factory :shipment, class: Spree::Shipment do
    # keeps test shipments unique per order
    initialize_with { Spree::Shipment.find_or_create_by(order_id: order.id) }

    tracking 'U10000'
    number '100'
    cost 100.00
    state 'pending'
    order
    address
    stock_location

    after(:create) do |shipment, evalulator|
      shipment.add_shipping_method(create(:shipping_method), true)

      shipment.order.line_items.each do |line_item|
        line_item.quantity.times { shipment.inventory_units.create(variant_id: line_item.variant_id) }
      end
    end
  end

  factory :shipment_with, class: Spree::Shipment do
    tracking 'U10000'
    number '100'
    cost 100.00
    state 'pending'
    order
    address
    stock_location

    trait :shipping_method do
      transient do
        shipping_method { create(:shipping_method) }
      end

      shipping_rates {
        [Spree::ShippingRate.create(shipping_method: shipping_method, selected: true)]
      }

      after(:create) do |shipment, _evaluator|
        shipment.order.line_items.each do |line_item|
          line_item.quantity.times {
            shipment.inventory_units.create(variant_id: line_item.variant_id)
          }
        end
      end
    end
  end
end
```
assistant