var atype = 'digital'

var activity_uuid = 'a039z8f9-24e5-495c-a894-a044c767c49a'

var body2 = {
  "data": {
    "type": "my_activity--"+atype,
    "id": activity_uuid,
    "attributes": {
      'id': 18,
      'uuid': activity_uuid,
      'notes': 'some debug text if you have a notes field'
    },
    "relationships": {
      "type": {
        "data": {
          "type": "my_activity_type--my_activity_type",
          "id": "109e0f74-1995-4836-a044-7fc40f05b39c"
        },
        "links": {
          "self": "http://me.net/my_activity/digital/"+activity_uuid+"/relationships/type",
          "related": "http://me.net/my_activity/digital/"+activity_uuid+"/type"
        }
      },
      "products": {
        "data": [
          {
            "type": "paragraph--products",
            "id": "42c0aba4-0e5c-411d-99e0-63305b66c1ef",
            "meta": {
              "target_revision_id": "11" // A revision of this product.
            }
          },
          {
            "type": "paragraph--products",
            "id": "3da8f866-346e-4386-acc5-18040949828e",
            "meta": {
              "target_revision_id": "13"
            }
          }
        ],
        "links": {
          "self": "http://me.net/my_activity/digital/"+activity_uuid+"/relationships/products",
          "related": "http://me.net/my_activity/digital/"+activity_uuid+"/products"
        }
      }
    }

  }
}

try {

  init = {
    method: 'PATCH',
    headers: {
      'Authorization': 'Bearer '+auth.access_token,
      'Content-Type': 'application/vnd.api+json',
    },
    body: JSON.stringify(body2),
  };
  // Some fetch implementation.
  _fetch(this, '/my_activity/'+atype+'/'+activity_uuid, init)

} catch(error) {
  // Something.
}