Stuffing maps into DynamoDB
posted 2020.10.02 by Clark Wilkins, Simplexable

The following is a simple way to (a) instantiate an object (map) in a document record if it's not in place yet, or (b) add a dependent object (map) to an existing object in the record. This gets around problems with updating 

documentId.views.thisUser...
where documentId does not have a views.thisUser attribute yet.

// establish the route for Express.js

router.post("/log-view", async (req, res) => {
  const { itemId, itemType, userId } = req.body;


// itemType is determining which DDB table I am working on here...

  if (itemType === 3) {
    params = {
      TableName: "healthica-sales-discussions",
      KeyConditionExpression: "#i = :i",
      ExpressionAttributeNames: { "#i": "id", "#v": "views" },
      ExpressionAttributeValues: { ":i": itemId },
      ProjectionExpression: "#v",
    };
  }


// getData gets the document record using the parameters above...

var data = await getData(params, res, "returnData");
  var { Items } = data;
  var viewTime = new Date().toUTCString();


// If my main counter field is not there, the entire object
// (aka "map") is not valid, so define it

  var views = Items[0].views || { total: 0 };
  if (isNaN(views.total)) {
    views = { total: 0 };
  }


// This is a backup in case the total field has been corrupted

  if (isNaN(views.total)) {
    views = { total: 0 };
  }


// If I have a view count for this user, increment by one.
// Otherwise, let's start at 1.

  try {
    var userViews = views[userId].total + 1;
  } catch (err) {
    var userViews = 1;
  }
  

// Total views always increments by one.

  views.total += 1;
  views[userId] = { total: userViews, latest: viewTime };


// Now update where the entire views object is replaced.

  params = {
    TableName: "healthica-sales-discussions",
    Key: { id: itemId },
    ExpressionAttributeNames: { "#v": "views" },
    ExpressionAttributeValues: { ":v": views },
    UpdateExpression: "SET #v = :v",
  };
  updateRecord(params, res);
});