script.liquid

{% if event.preview %}
  {
    "action": {
      "type": "shopify",
      "options": [
        "post",
        "/admin/inventory_levels/set.json",
        {
          "inventory_item_id": 1234567890,
          "location_id": {{ shop.primary_location_id | json }},
          "available": {{ options.inventory_level_to_apply__number_required | round | json }}
        }
      ]
    }
  }
{% else %}
  {% assign collection_id = options.collection_id__number_required | round %}
  {% assign collection = shop.collections[collection_id] %}

  {% unless collection %}
    {% capture message %}No collection found with ID #{{ collection_id | json }}.{% endcapture %}
    {"log": {{ message | json }}}
  {% endunless %}

  {% if collection.products.count == 0 %}
    {% capture message %}No products in collection #{{ collection_id | json }}.{% endcapture %}
    {"log": {{ message | json }}}
  {% endif %}

  {% for product in collection.products %}
    {% for variant in product.variants %}
      {% if variant.inventory_management != "shopify" %}
        {% capture message %}Inventory management policy is {{ variant.inventory_management | json }}, skipping inventory update for {{ variant.title }} on {{ product.title }}{% endcapture %}
        {"log": {{ message | json }}}
        {% continue %}
      {% endif %}

      {% for inventory_level in variant.inventory_levels %}
        {% if inventory_level.location_id != shop.primary_location_id %}
          {% continue %}
        {% endif %}

        {% if inventory_level.available == options.inventory_level_to_apply__number_required %}
          {% capture message %}Inventory level is already {{ inventory_level.available | json }}, skipping inventory update for {{ variant.title }} on {{ product.title }}{% endcapture %}
          {"log": {{ message | json }}}
          {% continue %}
        {% endif %}

        {
          "action": {
            "type": "shopify",
            "options": [
              "post",
              "/admin/inventory_levels/set.json",
              {
                "inventory_item_id": {{ inventory_level.inventory_item_id | json }},
                "location_id": {{ inventory_level.location_id | json }},
                "available": {{ options.inventory_level_to_apply__number_required | round | json }}
              }
            ]
          }
        }
      {% endfor %}
    {% endfor %}
  {% endfor %}
{% endif %}