script.liquid

{% assign customer_metafields_and_order_attributes = options.customer_metafields_and_order_attributes__keyval_required %}

{% capture query %}
  query {
    order(id: {{ order.admin_graphql_api_id | json }}) {
      id
      customer {
        metafields(
          first: {{ customer_metafields_and_order_attributes.size }}
          keys: {{ customer_metafields_and_order_attributes | keys | json }}
        ) {
          nodes {
            key
            value
          }
        }
      }
      customAttributes {
        key
        value
      }
    }
  }
{% endcapture %}

{% assign result = query | shopify %}

{% if event.preview %}
  {% capture result_json %}
    {
      "data": {
        "order": {
          "id": "gid://shopify/Order/1234567890",
          "customer": {
            "metafields": {
              "nodes": [
                {
                  "key": {{ customer_metafields_and_order_attributes | first | first | json }},
                  "value": "Alpha"
                }
              ]
            }
          },
          "customAttributes": [
            {
              "key": "__sample",
              "value": "Beta"
            }
          ]
        }
      }
    }
  {% endcapture %}

  {% assign result = result_json | parse_json %}
{% endif %}

{% assign metafields = result.data.order.customer.metafields.nodes %}
{% assign attributes = result.data.order.customAttributes | default: array %}

{% log
  existing_attributes_on_order: attributes,
  matched_customer_metafields: metafields
%}

{% for metafield in metafields %}
  {% assign attribute = hash %}
  {% assign attribute["key"] = customer_metafields_and_order_attributes[metafield.key] %}
  {% assign attribute["value"] = metafield.value %}
  {% assign attributes = attributes | push: attribute %}
{% else %}
  {% break %}
{% endfor %}

{% action "shopify" %}
  mutation {
    orderUpdate(
      input: {
        id: {{ order.admin_graphql_api_id | json }}
        customAttributes: {{ attributes | graphql_arguments }}
      }
    ) {
      order {
        id
        name
        customAttributes {
          key
          value
        }
      }
      userErrors {
        field
        message
      }
    }
  }
{% endaction %}