Build a reactive publication with additional fields in each document

Solution 1:

It's relatively easy to keep fields private even if they are part of the database query. The last argument to self.added is the object being passed to the client, so you can strip/modify/delete fields you are sending to the client.

Here's a modified version of your fiddle. This should do what you are asking for. (To be honest I'm not sure why you had anything chained after the observeChanges function in your fiddle, so maybe I'm misunderstanding you, but looking at the rest of your question this should be it. Sorry if I got it wrong.)

var self = this;

// Modify the document we are sending to the client.
function filter(doc) {
  var length = doc.item.length;

  // White list the fields you want to publish.
  var docToPublish = _.pick(doc, [
      'someOtherField'
  ]);

  // Add your custom fields.
  docToPublish.itemLength = length;

  return docToPublish;                        
}

var handle = myCollection.find({}, {fields: {item:1, someOtherField:1}})
            // Use observe since it gives us the the old and new document when something is changing. 
            // If this becomes a performance issue then consider using observeChanges, 
            // but its usually a lot simpler to use observe in cases like this.
            .observe({
                added: function(doc) {
                    self.added("myCollection", doc._id, filter(doc));
                },
                changed: function(newDocument, oldDocument)
                    // When the item count is changing, send update to client.
                    if (newDocument.item.length !== oldDocument.item.length)
                        self.changed("myCollection", newDocument._id, filter(newDocument));
                },
                removed: function(doc) {
                    self.removed("myCollection", doc._id);                    
                });

self.ready();

self.onStop(function () {
  handle.stop();
});

Solution 2:

To solve your first problem, you need to tell MongoDB what fields it should return in the cursor. Leave out the fields you don't want:

MyCollection.find({}, {fields: {'a_field':1}});

Solving your second problem is also pretty easy, I would suggest using the collection helpers packages. You could accomplish this easily, like so:

// Add calculated fields to MyCollection.
MyCollection.helpers({
  item_count: function() {
    return this.items.length;
  }
});

This will be run before an object is added to a cursor, and will create properties on the returned objects that are calculated dynamically, not stored in MongoDB.