mongoose is removing empty object out of embedded documents in array - node.js

Question

notice the following code, that shows a schema with 2 arrays, one is configured to be from Type:
[
mongoose.Schema.Types.Mixed
]
and one is configured to be from type:
[
{
value: mongoose.Schema.Types.Mixed
}
]
Here is the code:
var mongoose = require('mongoose');
mongoose.connect('mongodb://localhost/test');
var schema = new mongoose.Schema({
withSchema: [{
value:mongoose.Schema.Types.Mixed}
],
withoutSchema: [mongoose.Schema.Types.Mixed],
} , {minimize: false});
var Tweak = mongoose.model('tweak', schema );
I update the document using the same data:
var data = {
"withSchema" : [ { "value": { a:"221", b:{} } } ],
"withoutSchema" : [ { "value": { a:"221", b:{} } } ]
}
Tweak.findByIdAndUpdate("545680170960023a185ea77e", data, function(err, doc){
console.log(doc);
//{
// "withSchema" : [ { "value": { a:"221" } } ],
// "withoutSchema" : [ { "value": { a:"221", b:{} } } ]
//}
});
How do I prevent this b:{} removal?
EDIT:
It turns out this happens only when there is an embeddedDocument inside an Array.

Answer 1

Removal of empty objects from arrays - is cause by the minimize option of schema - which defaults to 'true'. Erken answered this in a comment in the down voted answer above - putting it as a separate answer so people can find it.
Can be overridden to 'false' in the schema - then it will save empty objects in arrays
var schema = new Schema({ name: String, inventory: {} }, { minimize: false });
from http://mongoosejs.com/docs/guide.html#minimize

Answer 2

This approach involves first retrieving the document from Mongo, then issuing an update command (triggered by calling save). Take a look at the following code.
var id = "54619b5ef610b70b14a46e79";
Tweak.findById(id, function(err, result) {
if (err) {
console.log(err);
}
console.log(result.withSchema[0].value);
result.withSchema[0].value = data.withSchema[0].value;
result.withoutSchema.value = data.withoutSchema.value;
result.save(function(err) {
if (err) {
console.log(err);
}
console.log('updated');
});
});
After saving the document run the code snippet with appropriate 'id' value.

Answer 3

Objects which evaluate to null (as your b does) are skipped by mongoose. As an empty or non existant object evaluates to null in all MongoDB drivers and even shell queries, too, it actually does not make a difference wether an empty b is saved or not.
For example, if you query if b exists, since b is empty, it evaluates to nulland hence the query would fail for that document, regardless if bhas an empty value or simply isn't there.
Since you already use the minimize option, I assume that the empty result, since it evaluates to null, simply isn't displayed. When checking with the shell, the key should be there with an empty value.
Bottom line: for all practical purposes, it does not make a difference wether b holds an empty value or simply isn't there.