The outcome of accessing MongoDB in Meteor JS is influenced by the timing of the access

In my Meteor JS app, I am working with a collection called MenuItems:

MenuItems = new Mongo.Collection('menu_items');

Within my Template Helper file, I have defined a function to access this collection:

Template.admin_menu_items.helpers({

  menuItems: function(){
    //return items from DB
    console.log('inside menuItems');

    snapshot = MenuItems.find().fetch();

    console.log(snapshot);

    return snapshot;

  },
});

I then utilize this helper in my html template file as follows:

{{#each menuItems}}
  {{#each items}}
    {{this}}
  {{/each}}

{{/each}}

Additionally, in the Template rendered helper callback section, I also retrieve data from the same collection:

Template.admin_menu_items.rendered = function(){

  console.log('Template.admin_menu_items.rendered');

  var snapshotRendered = MenuItems.find().fetch();
  
  console.log(snapshotRendered);


}

Upon page refresh, the output displays some interesting results regarding the fetched data:

inside menuItems                    admin_m...4d3e6ec (line 9)
[]                                  admin_m...4d3e6ec (line 13)
Template.admin_menu_items.rendered  admin_m...4d3e6ec (line 36)
[]                                  admin_m...4d3e6ec (line 40)
inside menuItems                    admin_m...4d3e6ec (line 9)
[Object { _id="nHZBfwAt64dwiPjCB",  items=[3]}]

I am seeking clarity on why the initial calls to MenuItems.find().fetch() yield empty arrays despite having documents within the collection. Could this be attributed to the loading sequence of files? How can I ensure that MongoDB returns the correct number of documents right from the first call?

Your insights and suggestions would be greatly appreciated.

Answer №1

Subscriptions and data transmission play a larger role here compared to the file load order. When you refresh the page, the template is loaded first followed by the data, leading to a series of operations like:

  1. The template begins building without any data, causing the helper to run with an empty array
  2. Once the template is fully built, it renders with an empty array due to lack of data
  3. Eventually, the data arrives
  4. Upon receiving the new data, the helper re-runs reactively and returns a complete array

To handle this situation, particularly if you require the data in the rendered callback, you can delay the template execution until the data is received. This can be achieved through a router that waits for a subscription, or by checking the data availability before rendering the template, or by using Template.subscribe function along with an if statement to guard the template until the data is ready.

Additionally, consider returning the result of find() in your helper rather than using find().fetch(). While both options work similarly within an {{#each}} block, returning the cursor allows for more precise reactivity as Blaze can iterate over it directly, unless there is a specific need for an array.

Answer №2

How can I guarantee that MongoDB will provide the correct number of documents right from the initial call or access made to it???

Well, in line with the principles of Meteor... You cannot.

The essence of Meteor is reactivity. You do not want to be required to wait for data to arrive before rendering begins. This would result in the user staring at a blank page for a considerable amount of time before any action appears, because the page halts rendering, trapped in darkness while awaiting the complete dataset.

Instead, opt for reactivity. Utilize reactive computations which are triggered instantly and then whenever the reactive data within them is updated. Helpers function as reactive computations, while Mongo cursors in Meteor act as reactive data sources. Helpers run initially (potentially returning nothing if the data has not arrived yet), and update each time new data is received.

It is important to consider the scenario of "No data available yet" when writing these reactive computations. For instance, using if(someCollection.findOne()) ensures that there is at least one document present in someCollection.
Although in your specific case, this may not be necessary as Spacebars handles cursors effectively.

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