Mongo query array.length within an array - mongodb

Question

I tried to use length on an array that is within another array and got an error:
db.quotes.find( { $where: "this.booksWithQuote.authors.length>0" } )
// fails with this.booksWithQuote.authors is undefined
I'm basing this syntax on this example MongoDB – find all documents where an array / list size is greater than N that works:
db.domain.find( {booksWithQuote: {$exists:true}, $where:'this.booksWithQuote.length>0'} )
// Above works
So I'm wondering if this is possible, how can I find all documents that have array1.array2 where array2 length is greater than zero.
I've tried the following but it fails with a syntax error:
db.quotes.find( {
"booksWithQuote": {$exists: true} },
"booksWithQuote.authors": {$exists: true} },
$where: "booksWithQuote.authors.length>0" } )
// fails with this.booksWithQuote.authors is undefined
It's worth pointing out that if I knew the author of a book, the nested array with array searching worked. Pretty cool!
db.quotes.find( { "booksWithQuote.authors" : "Sam Fisher" } )
// Returns all quotes that have a book that has the author Sam Fisher
But in my case I'm just trying to find all of the quotes that have more than one author on any given book.
To follow along, consider this example.
I have a collection of quotes, and each quote has a list of books where the quote was used, and each book has an array of authors.
Below is some sample data so you can understand the structure of the data. It shows one quote with no books, another quote with a book but no authors, and a third quote with books and several authors.
[
{
quoteText: "Love to code",
booksWithQuote: [ ]
},
{
quoteText: "Another Quotesake",
booksWithQuote: [
{
title: "Where is elmo",
authors: [ ]
}
]
},
{
quoteText: "For goodness sake",
booksWithQuote: [
{
title: "The search for Elmo",
authors: [
"John Smith",
"Sam Fisher",
"Jim Wiggins"
]
},
{
title: "Finding Elmo",
authors: [ "Sam Fisher" ]
},
{
title: "Mercy me",
authors: [ ]
}
]
}
]
So to reiterate, How can I find all documents that have an array within another array where the 2nd array has one or more elements?

Answer 1

You can use $exists operator and refer to first element of an array using dot notation:
db.col.find({ "booksWithQuote.authors.0": { $exists: true } })

Answer 2

MongoDB documentation: https://www.mongodb.com/docs/manual/reference/operator/query/expr/
you can use $expr to achieve this
$expr:{$gte: [{$size: "$selectedArray"}, size]}