Skip one nested level of subdocument in Mongoose - node.js

Question

I have a parent schema with a subdocument. The subdocument has a property with an array of embedded objects:
Child schema
var snippetSchema = new Schema({
snippet: [
{
language: String,
text: String,
_id: false
}
]
});
Parent schema
var itemSchema = new Schema({
lsin: Number,
identifier: {
isbn13: Number,
},
title: snippetSchema,
});
Which upon Item.find returns an object like so:
[
{
_id: (...),
lsin: 676765,
identifier: {
isbn13: 8797734598763
},
title: {
_id: (...),
snippet: [
{
language: 'se',
text: 'Pippi Långstrump'
}
]
}
}
]
I would like to skip one nested level of the subdocument when the object is returned to the client:
[
{
_id: (...),
lsin: 676765,
identifier: {
isbn13: 8797734598763
},
title: {
language: 'se',
text: 'Pippi Långstrump'
}
}
]
So far I have tried:
#1 using a getter
function getter() {
return this.title.snippet[0];
}
var itemSchema = new Schema({
...
title: { type: snippetSchema, get: getter }
});
But it creates an infinite loop resulting in RangeError: Maximum call stack size exceeded.
#2 using a virtual attribute
var itemSchema = new Schema({
..., {
toObject: {
virtuals: true
}
});
itemSchema
.virtual('title2')
.get(function () {
return this.title.snippet[0];
});
Which will generate the desired nested level but under a new attribute, which is not acceptable. To my knowledge there is no way of overriding an attribute with an virtual attribute.
The question is: is there any other way to go about in getting the desired output? There will be several references to the snippetSchema across the application and a DRY method is preferred.
I am new to MongoDB and Mongoose.

Answer 1

You'll need to use the $project within an mongodb aggregation pipeline.
Within my database I have the following:
> db.items.find().pretty()
{
"_id" : 123,
"lsin" : 676765,
"identifier" : {
"isbn13" : 8797734598763
},
"title" : {
"_id" : 456,
"snippet" : [
{
"language" : "se",
"text" : "Pippi Långstrump"
}
]
}
}
Then we just need to create a simple aggregation query:
db.items.aggregate([
{$project: { lsin: 1, identifier: 1, title: { $arrayElemAt: [ '$title.snippet', 0 ] }}}
])
This just uses a $project (https://docs.mongodb.com/v3.2/reference/operator/aggregation/project/) and a $arrayElemAt (https://docs.mongodb.com/v3.2/reference/operator/aggregation/arrayElemAt/) to project the first item out of the array. If we execute that we will get the following:
{
"_id" : 123,
"lsin" : 676765,
"identifier" : {
"isbn13" : 8797734598763
},
"title" : {
"language" : "se",
"text" : "Pippi Långstrump"
}
}