Yii2的mongodb集合 - 获取嵌套数组的所有内容

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