mongodb, check if the key exist while appending to an array - node.js

Question

Note: checking if the key books exist or not, creating if not and than updating it.
I am using mongodb driver with nodejs.
In the db.collection('userData')The document looks like this:
{
user_id: 'user1',
books: [{
id: 'book1',
title: 'this is book1'
},
{
id: 'book1',
title: 'this is book1'
}]
}
when inserting a new book entry, how to check if the array of books exists in the document, if not then add a key books in the document and then insert the book entry.

Answer 1

You have to do 2 separate queries,
Find user document
Check condition if books field present
If Present then push object, else set new field
var user_id = "user1";
var bookData = { id: 'book1', title: 'this is book1' };
// FIND USER DATA
var userData = await db.collection('userData').findOne({ user_id: user_id }, { books: 1 });
var updateBody = { $push: { books: bookData } };
// IF BOOKS FIELD NOT PRESENT THEN SET NEW
if (!userData.books) {
updateBody = { $set: { books: [bookData] } };
}
var updateData = await db.collection('userData').updateOne({ user_id: user_id }, updateBody);
console.log(updateData);
Second option you can use update with aggregation pipeline starting from MongoDB 4.2,
$ifNull check is field is null then return []
$concatArrays to concat current books with new book object
var bookData = { id: 'book1', title: 'this is book1' };
db.collection('userData').update({
// put your condition
},
[{
$set: {
books: {
$concatArrays: [
{ $ifNull: ["$books", []] },
[bookData]
]
}
}
}],
{ multi: true }
);
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