Yii2's mongodb collection - get all content of nested arrays - php

Question

I have a Yii2 mongodb's collection that looks like this:
{
"_id" : "123",
"books" : [
{
"author" : "John",
"title" : "The Book"
},
{
"author" : "Smith",
"title" : "Something!"
}
]
}
{
"_id" : "321",
"books" : [
{
"author" : "John",
"title" : "The Book"
}
]
}
...
And I want to get an array of all books (an array of arrays basically):
[
{
"author" : "John",
"title" : "The Book"
},
{
"author" : "Smith",
"title" : "Something!"
},
{
"author" : "John",
"title" : "The Book"
}
...
]
I saw answers to close questions, but they all achieve something a bit different.
Also tried $collection->distinct('books', [], []) and it worked, but it removed duplicates which is unacceptable.

Answer 1

Use this MongoDB query to get this resultset
db.collection.aggregate([
{ $unwind : "$books"},
{ $group: {
_id: null,
items:
{ $push: "$books" }
}}
]);

Answer 2

Let's try like this way using foreach()?
<?php
$json = '[{"_id":"123","books":[{"author":"John","title":"The Book"},{"author":"Smith","title":"Something!"}]},{"_id":"321","books":[{"author":"John","title":"The Book"}]}]';
$array = json_decode($json, 1);
$ids = [];
foreach($array as $v) {
foreach($v['books'] as $book){
$books[] = $book;
}
}
echo json_encode($books);
?>
Output:
[{"author":"John","title":"The Book"},{"author":"Smith","title":"Something!"},{"author":"John","title":"The Book"}]
DEMO: https://3v4l.org/hdJ8H