Where do I set Session defaults so that they are available in my subscriptions? - meteor

Question

I have a helper function that depends on a collection document lookup, the result of which it passes to a subscription via a Session. It then needs to query the documents from that subscription.
The code explains it better than I could.
Helper:
var selection = Selections.findOne()
var itemIds = function() {
return selection && selection.itemIds
}
var itemIdsArray = itemIds()
Session.set('itemIdsArray', itemIdsArray)
console.log(Session.get('itemIdsArray'))
_.each(Items.find({_id: {$in: itemIdsArray}}).fetch(), function(element, index, list) {
//doing stuff
})
Subscription:
Meteor.subscribe('itemsById', Session.get('itemIdsArray'))
Publication:
Meteor.publish('itemsById', function(itemIdsArray) {
return Items.find({_id: {$in: itemIdsArray}})
})
My console.log returns an undefined value before it returns the array of IDs. So undefined gets passed all the way to the publication, which complains of a null value (which is weird in itself) after $in and breaks.
My solution was to set the Session to default to [],
Session.setDefault(`itemIdsArray`, [])
which I honestly had high hopes that it'd work, but alas, it did not.
I've tried putting it inside IronRouter's onBeforeAction, I've tried putting it at the top of the helper, I've tried putting it pretty much anywhere but it still logs and returns undefined once before it gets the correct value.
I've also tried to move around my subscription, from waitOn to subscriptions to onAfterAction to onRendered, but those attempts have been utterly fruitless.
What should I do?

Answer 1

That's fairly typical behavior in Meteor. Session variables are not always ready at the time. The usual way of dealing with this is to introduce a guard in the helper that checks the variable is defined before doing anything else with it.
In your case something like this would work: itemsIdArray = itemIds() || [];
To answer the actual question you are asking, where do you set Session defaults that they are available in your subscriptions: it's not important where you set them, but when you access them. You can wait for the subscription to be ready using iron router's waitOn() function, or you can check the subscription handle's ready() function (see https://github.com/oortcloud/unofficial-meteor-faq#user-content-how-do-i-know-when-my-subscription-is-ready-and-not-still-loading)

Answer 2

If you return a subscription in your waitOn option of Iron Router you should have the data in your template then:
Router.route('/yourRoutePath/:_id', {
// this template will be rendered until the subscriptions are ready
loadingTemplate: 'loading',
waitOn: function () {
// return one handle, a function, or an array
return Meteor.subscribe('itemsById', this.params._id);
},
action: function () {
this.render('myTemplate');
}
});
Your template helper:
Template.myTemplate.helpers({
items: function() {
return Items.find();
}
});
I noticed that you publish Items collection and you want to use Selections collection in your helper. If you need more than one subscription, you can return an array of subscriptions in waitOn:
waitOn: function () {
// return one handle, a function, or an array
return [
Meteor.subscribe('itemsById', this.params._id),
Meteor.subscribe('selections')
];
}
WaitOn ensures that your template will be rendered when all subscriptions are ready.