Meteor - MongoDB Error: Cannot apply $addToSet modifier to non-array - mongodb

Question

I am following the recently published book "Getting Started with Meteor.js JavaScript Framework" by Isaac Strack. The book works with Meteor 0.5.0. I am working with version 0.5.4.
In the book you build an app with a few categories to which you insert data for tracking household items, and who they may be lent to. I deployed the app to a meteor subdomain, and it is working perfectly. It does not replicate my local MongoDB error.
I am in Chapter 5, and I have just removed autopublish from the app, and specified my local channels for data.
Locally, under the under the "Tools" category only, when I try to add a new item to the category, I recieve this error in my browser console:
Exception while simulating the effect of invoking '/Lists/update' Error {} Error: Cannot apply $addToSet modifier to non-array
at Error (<anonymous>)
at LocalCollection._modifiers.$addToSet (http://localhost:3000/packages/minimongo/modify.js?e7f02f0df0bff9f0b97236f9548637b7ede1ac74:178:13)
at Function.LocalCollection._modify (http://localhost:3000/packages/minimongo/modify.js?e7f02f0df0bff9f0b97236f9548637b7ede1ac74:53:9)
at LocalCollection._modifyAndNotify (http://localhost:3000/packages/minimongo/minimongo.js?7f5131f0f3d86c8269a6e6db0e2467e28eff6422:474:19)
at LocalCollection.update (http://localhost:3000/packages/minimongo/minimongo.js?7f5131f0f3d86c8269a6e6db0e2467e28eff6422:444:12)
at m.(anonymous function) (http://localhost:3000/packages/mongo-livedata/collection.js?3ef9efcb8726ddf54f58384b2d8f226aaec8fd53:415:36)
at http://localhost:3000/packages/livedata/livedata_connection.js?367884963b120d457819216ff713b2586b266dde:540:25
at _.extend.withValue (http://localhost:3000/packages/meteor/dynamics_browser.js?46b8d1f1158040fcc2beb7906ec2f932871a398d:21:19)
at _.extend.apply (http://localhost:3000/packages/livedata/livedata_connection.js?367884963b120d457819216ff713b2586b266dde:539:47)
at Meteor.Collection.(anonymous function) [as update] (http://localhost:3000/packages/mongo-livedata/collection.js?3ef9efcb8726ddf54f58384b2d8f226aaec8fd53:266:23) logging.js:30
update failed: Internal server error logging.js:30
The tools category already has one item in it which was submitted earlier in the tutorial. If I type into the console lists.findOne({Category:"Tools"}); I get the output which recognizes an item in the Object:
Object
Category: "Tools"
_id: "eaa681e1-83f2-49f2-a42b-c6d84e526270"
items: Object
LentTo: "Steve"
Name: "Linear Compression Wrench"
Owner: "me"
__proto__: Object
__proto__: Object
However, the screen output is blank:
Naturally I have tried restarting the meteor server & shut down the browser, but no resolution. I am new to MongoDB, so I am unclear as to where to turn to understand what is causing this problem, or why.
You can view the app here. You can view the code on my GitHub.

Answer 1

function addItem(list_id, item_name) {
if(!item_name && !list_id)
return;
lists.update({_id:list_id}, {$addToSet:{items:{Name:item_name}}});
}
Seems like you're trying to add an object to a set. You're getting an error on simulation. Let's investigate that error. The code that errors out:
https://github.com/meteor/meteor/blob/master/packages/minimongo/modify.js
$addToSet: function (target, field, arg) {
var x = target[field];
if (x === undefined)
target[field] = [arg];
else if (!(x instanceof Array))
throw Error("Cannot apply $addToSet modifier to non-array");
else { ...
Uh oh, throw Error("Cannot apply $addToSet modifier to non-array.").
Look at your code:
Object
Category: "Tools"
_id: "eaa681e1-83f2-49f2-a42b-c6d84e526270"
...
items: Object
...
items is an object, not an array! It will error out.
Can you $addToSet to an object with Mongo? Let's look at the code.
https://github.com/mongodb/mongo/blob/4a4f9b1d6dc79d1ba4a7d7eaa9e4eb6d00aa466c/db/update.cpp
case ADDTOSET: {
uassert( 12592 , "$addToSet can only be applied to an array" , in.type() == Array );
...
}
Nope! This is from old Mongo code, because the contemporary codebase is sprawling, but same thing.
I only found one insert in your code.
'keyup #add-category': function(e, t) {
if (e.which === 13) {
var catVal = String(e.target.value || "");
if (catVal) {
lists.insert({Category:catVal});
Session.set('adding_category', false);
}
}
},
Try lists.insert({Category:catVal,items:[]}). So that items is initialized as an array rather than an object when it was first used.
Also, I don't think $addToSet compares objects in an array the way you would like anyway, so consider making a separate collection Items that contains a categoryId.
It is purely a coincidence that it is working on one place and not another.

Answer 2

I had the same problem following the same guide. When using the inserts directly from the guide (the Tools and DVDs) minimongo inserts the items as objects instead of arrays. I don't know if it was simply a mistake or if the code base has changed, either way I changed:
lists.insert({Category:"DVDs", items: {Name:"Mission Impossible", Owner:"me", LentTo:"Alice"}});
to
lists.insert({Category:"DVDs", items: [{Name:"Mission Impossible", Owner:"me",LentTo:"Alice"}]});
just encasing the items in [] to force it to an array and did likewise with the insert for tools. Works great now.