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.