script.liquid

{% assign existing_metafield = fulfillment.order.metafields.mechanic | where: "key", "known_tracking_numbers" | first %}

{% if event.preview %}
  {% capture fulfillment_json %}
    {
      "order": {
        "email": "customer@example.com",
        "metafields": {
          "mechanic": {
            "known_tracking_numbers": []
          }
        },
        "fulfillments": [
          {
            "tracking_numbers": ["ABC123"]
          }
        ]
      }
    }
  {% endcapture %}

  {% assign fulfillment = fulfillment_json | parse_json %}
            
  {% assign existing_metafield = hash %}
  {% assign existing_metafield["id"] = 1234567890 %}
{% endif %}
            
{% assign already_known_tracking_numbers = fulfillment.order.metafields.mechanic.known_tracking_numbers | default: array %}

{% assign currently_known_tracking_numbers = array %}
{% for some_fulfillment in fulfillment.order.fulfillments %}
  {% assign currently_known_tracking_numbers = currently_known_tracking_numbers | concat: some_fulfillment.tracking_numbers %}
{% endfor %}

{% assign tracking_numbers_changed = false %}
{% for tracking_number in currently_known_tracking_numbers %}
  {% unless already_known_tracking_numbers contains tracking_number %}
    {% assign tracking_numbers_changed = true %}
    {% break %}
  {% endunless %}
{% endfor %}

{% if tracking_numbers_changed or existing_metafield == nil %}
  {% if existing_metafield %}
    {% if fulfillment.order.email == blank %}
      {% log "No email on file for this order." %}
    {% else %}
      {% assign tracking_numbers_string = currently_known_tracking_numbers | join: ", " %}
      {% action "email" %}
        {
          "to": {{ fulfillment.order.email | json }},
          "subject": {{ options.email_subject__required | json }},
          "body": {{ options.email_body__required_multiline | replace: "TRACKING_NUMBERS", tracking_numbers_string | strip | newline_to_br | json }},
          "reply_to": {{ shop.customer_email | json }},
          "from_display_name": {{ shop.name | json }}
        }
      {% endaction %}
    {% endif %}
  {% endif %}

  {% action "shopify" %}
    [
      "update",
      [
        "order",
        {{ fulfillment.order.id | json }}
      ],
      {
        "metafields": [
          {
            {% if existing_metafield %}"id": {{ existing_metafield.id | json }},{% endif %}
            "namespace": "mechanic",
            "key": "known_tracking_numbers",
            "value": {{ currently_known_tracking_numbers | json | json }},
            "type": "json"
          }
        ]
      }
    ]
  {% endaction %}
{% endif %}