如何对嵌入文档进行聚合查询？

Question

I am using jenssegers/laravel-mongodb library in a laravel application however I need to show counts of an embedded document. Using a generic example of comment/posts, while I can solve my problem by just pulling all the posts and looping through to get comments to count them but was just was not sure if I could query them.

I did set up my relationships. In my post class I did:

 public function comments()
    {
        return $this->hasMany('App\Comment');
    }

and in my comment class:

 public function post()
        {
            return $this->belongsTo('App\Post');
        }

Later in code:

 $post->comments()->save($comment);
 $comment->post()->associate($post);

my document structure:

"posts" : [ 
{
    "_id" : ObjectId("5805a11e2594ee26543ea041"),
    "Post_Num" : "166236001010",
    "updated_at" : ISODate("2016-10-18T04:12:14.454Z"),
    "created_at" : ISODate("2016-10-18T04:12:14.451Z"),
    "comments" : [ 
        {   
            "Comment_Num" : "3333333",
            "_id" : ObjectId("5805a11e2594ee26543ea042"),
            "post_id" : "5805a11e2594ee26543ea041",
        },
        {   
            "Comment_Num" : "3333333",
            "_id" : ObjectId("5805a11e2594ee26543ea042"),
            "post_id" : "5805a11e2594ee26543ea041",
        }
    ]

},

{
    "_id" : ObjectId("5805a11e2594ee26543ea041"),
    "Post_Num" : "166236001010",
    "comments" : [ 
        {   
            "Comment_Num" : "3333333",
            "_id" : ObjectId("5805a11e2594ee26543ea042"),
            "post_id" : "5805a11e2594ee26543ea041",
        }
    ]

}
]

Now when I try getting the comments like:

$post->comments()->count() 
or
$post->comments()->get()->count() 
or
$post->comments->get()->count() 

I get a 0. The same logic works if it is not an embedded document but just was wondering if it was possible to do an aggregate query ? Perhaps is best to just let the code iterate and add everything?

As you can tell I need some minor hand holding. Thank You

UPDATE: I am trying the following

 public function commentCount()
    {

        $commentsCount = Post::raw(function($collection)
        {

            return $collection->aggregate(['$project' => ['_id' => 1,
                'comments_count' => ['$size' => '$comments']],
                ['$group' => ['_id' => null, 'count' => ['$sum' => '$comments_count']]]]);

        });
        return $commentsCount;
    }

What I get now is:

$pipeline is not a list (unexpected index: "$project") 

Answer 1

Just to be clear, you want a list of your posts with the number of comments on each post?

Aggregation has something to offer for that: https://docs.mongodb.com/manual/reference/operator/aggregation/size/#exp._S_size

I'm not a php dev but this is my shot at it:

Post::raw()->aggregate(
    ['$project' => ['_id' => 1,
                    'Post_Num' => 1,
                    'comments_count' => ['$size' => '$comments']],
    ['$group' => ['_id' => null, 'count' => ['$sum' => '$comments_count']]]
]);