Match documents with their inner array element variables in MongoDB - mongodb

Question

I can't understand how to compare a document variable to another document variable. My goal is to match all Authors who have at least one book written in their mothertongue (native language).
However, after unwinding the books array, My $match: { mothertongue: "$bookLang"}} doesn't return return anything, eventhough they're the same in the $project stage.
Can you help me without javascript?
This is my current query:
db.author.aggregate([
{
$unwind: "$books"
},
{
$project: {
books: true,
mothertongue: true,
bookLang: "$books.lang"
}
},
{
$match: { mothertongue: "$bookLang"}
}
])
And here is a sample of the dataset
{
"_id" : ObjectId("5aa7b34a338571a7470be0eb"),
"fname" : "Minna",
"lname" : "Canth",
"mothertongue" : "Finnish",
"birthdate" : ISODate("1844-03-19T00:00:00Z"),
"deathdate" : ISODate("1897-05-12T00:00:00Z"),
"books" : [
{
"title" : "Anna Liisa",
"lang" : "Finnish",
"language" : "finnish",
"edition" : 1,
"cover" : "Hard",
"year" : 1895,
"categorytags" : [
"Finland"
],
"publisher" : [
{
"name" : "Tammi",
"pubId" : ObjectId("5aa7b34a338571a7470be0e4")
}
]
},
{
"title" : "The Burglary and The House of Roinila",
"lang" : "English (UK)",
"translator" : ObjectId("5aa7b34a338571a7470be0ee"),
"cover" : "Soft",
"year" : 2010,
"categorytags" : [
"Finland"
],
"publisher" : [
{
"name" : "Jonathan Cape",
"pubId" : ObjectId("5aa7b34a338571a7470be0e7")
}
]
},
{
"title" : "Anna Liisa 2 ed.",
"lang" : "Finnish",
"language" : "finnish",
"edition" : 2,
"cover" : "hard",
"year" : 1958,
"categorytags" : [
"Finland"
],
"publisher" : [
{
"name" : "Otava",
"pubId" : ObjectId("5aa7b34a338571a7470be0e9")
}
]
}
]
}
End goal. note I'm not interested in formatting just yet, just the filtering
{
"Author" : "Charles Bukowski",
"BooksInMothertongue" : [
"Love Is a Dog from Hell"
]
}
{
"Author" : "Minna Canth",
"BooksInMothertongue" : [
"Anna Liisa",
"Anna Liisa 2 ed."
]
}
...

Answer 1

Try this
db.author.aggregate([{
$match: {
books: {
$ne: []
}
}
},
{
$project: {
books: {
$filter: {
input: "$books",
as: "book",
cond: {
$eq: ["$$book.lang", "$mothertongue"]
}
}
},
fname: 1
}
}, {
$unwind: "$books"
},
{
$group: {
_id: "$_id",
Author: {
$first: '$fname'
},
BooksInMothertongue: {
$push: "$books.title"
}
}
}
])